RUESSELSHEIM, WEST GERMANY -- A few days after Adam Opel A.G., the West German auto company owned by General Motors Corp., announced its joint venture with an East German carmaker, strangely familiar envelopes started flowing into Opel's executive offices here.

The letters came from individual East Germans and they were written on old Opel stationery. Very old.

The letters were requests from people who had been Opel dealers before World War II and Germany's post-war division. They had dug out the old stationery and written to Opel, asking if they could put their dealerships back in business.

Some of those dealerships are up and running again, part of East Germany's explosion of interest in Western cars.

Opel, for example, already has 171 dealers in East Germany. Fifteen teams of Opel executives are canvassing the country for additional sites, hoping to set up a 400-dealer service network in the next 18 months.

In the first months after the opening of the Berlin Wall, West Germany's automakers fell over each other in their rush to announce plans to form joint ventures and begin making parts or assembling cars in the East.

There were grand statements about reclaiming prewar market shares and satisfying a hungry new market of drivers. Opel said it would build 150,000 cars a year at the East German Wartburg plant in Eisenach.

But as the euphoria of 1989 eases, revealing the realism of 1990, auto companies are joining other manufacturers in putting the brakes on investment in the East until the country makes drastic changes in its laws.

Opel and its parent GM say they will start assembling 10,000 cars a year later this year at Eisenach, but the parts, including the bodies, will come mostly from plants in the West. Opel now says it cannot conceive of building any cars at Eisenach until 1994 at the earliest.

Auto executives remain confident, however, that East Germans want Western cars and that a vast new demand for vehicles in East Germany and other Eastern European countries will add to already burgeoning sales in a Western Europe that is rolling toward economic unity in 1992.

In the East German market, for example, a recent marketing survey in Leipzig found that 70 percent of East Germans hope to buy a car in the near future.

More than half want a Western model; 26 percent even named the Volkswagen Golf as their first choice.

The real and anticipated surge in demand already had led to a series of new combinations in Western Europe among the established names in European auto-making -- including longstanding and very profitable ventures by GM and Ford Motor Co. -- as well as a series of new plant announcements by Japanese automakers.

Still, even a population of car-hungry consumers who are about to have their life savings converted into hard Western cash is not enough to persuade VW, Opel, Mercedes and BMW to start investing in new plants -- not yet. Too much remains to be settled in eventual unity of the two parts of Germany, despite the July 1 date for economic union.

"We still don't know the rules," said Opel spokesman Jochen Kruse.

"Nobody knows what taxes will have to be paid there. Nobody knows what wages will be paid. We don't even know if we're going to be allowed to own our own plant," Kruse said.

"I have a stack of letters from people who claim to own the land under the plant" at Eisenach, said Joachim Beickler, Opel's controller.

"Obviously, we cannot make an investment until we have clear title to property, until we know the conditions under which we will employ people, until the social security law is clear," Beickler said.

The East German government has insisted in talks with West Germany that no Western company be allowed to own property in the East for several years after unification.

West German Chancellor Helmut Kohl personally argued to East German Prime Minister Lothar de Maiziere that the ability to own property is an essential prerequisite to Western investment in the country. The debate continues.

Opel and parent GM also are talking about deals in Poland, Czechoslovakia and the Soviet Union. They already have set up joint ventures in Hungary and East Germany.

The plants in the East are not only intended to serve new Eastern European markets, but also to expand Opel's insufficient capacity in the expanding Western European market and take advantage of what are expected to be favorable employment conditions.

Union Parity That is the catch. East Germany's old, Communist-run unions and West Germany's huge, powerful unions are both demanding that East German workers be treated exactly like their West German counterparts -- same wages, same time off, same benefits.

That is anathema to Opel, which is interested in East Germany in good measure because it promises to be a cheap place to make cars.

"We hope to gain considerable advantages in the East," Beickler said, "not from lower wages, but from more flexible work rules, Japanese-style team work, and less strict job classification.

"There is great risk that our rigid union regulations could be imposed in the East," Beickler said. "We'll have to see if the East Germans can protect their interest, which is to have a job."

Judging from the reaction to its announced joint venture, Opel executives are confident that East German workers will do whatever they have to do to get involved with a Western employer.

The company has been besieged with requests for jobs and dealerships.

Even before Western companies begin investing in the East, East German consumers are trying to dump their homemade Trabants and get behind the wheel of something -- anything -- Western.

"The East Germans want cars, videos and Marlboros," said Ulrich Ramm, chief of economic research at Commerzbank in Frankfurt.

At East Berlin's first used car lot devoted largely to Western models, East Germans can pick up a Fiat Panda for $6,200, payable, like everything else on the lot, only in West German marks.

Or they can splurge on an Audi 100 for a heady $13,100 -- almost twice what the average East German makes in a year.

Even at such prices, East Germans already have bought about 200,000 used Western cars this year, according to estimates by West German carmakers.

That's why the car companies are busy setting up service networks long before they build their first auto in the East.

Volkswagen is planning on a used car volume of 400,000 cars a year in East Germany alone -- a jump start that will boost both the used and new car markets in West Germany, analysts said.

Facing Bankruptcy The lot on Rummelsburger Landstrasse is actually part of one of East Germany's dying kombinats, the huge, state-run conglomerates that ran most business operations under the Communist system.

The IFA Kombinat, the same folks who make East Germany's sad little Trabant autos, set up the used car lot in an effort to collect some hard Western cash and help stay in business.

Like all state-run enterprises, IFA faces bankruptcy after July 1, when the deutsche mark becomes the currency of both Germanys, leaving East German businesses in the unenviable position of having to pay workers in hard cash while much of their income is in the form of unconvertible East Bloc currencies.

The used car business is going well so far, said director Helmut Genrich.

The kombinat buys cars from West German dealers and sells to customers who managed to collect West marks from West German relatives or trade their East-mark savings for hard cash.

Despite the apparent hardship involved in buying a Western car, so many East Germans already have done so that East Berlin police say they have seen sharp increases in both auto vandalism and, that most inevitable sign of Western urban life, car radio thefts.

On a building wall in the city's Prenzlauerberg section, someone has scrawled a massive graffito: "Stop the Car Madness! Travel by Bus and Rail!"

Genrich said IFA is stuck with hundreds of Trabants and Wartburgs, the East German autos that are still being produced, despite the sudden death of a market that until last fall created a 13-year waiting list for Trabants.

The only hope for the Trabants still being made is Romania, said West German bankers.

"The East German car industry was even behind Czechoslovakia," said Norbert Walter, chief economist at Deutsche Bank.

"They still have some chance to sell {Trabants} to Eastern markets, maybe Romania, but in the long run, they will have to give up. It's not that they were incapable. They just weren't creative," Walter said.

"Inevitably, there will be a shakeout in the East German industry," said Opel's Beickler.

Even the budding used car market probably will suffer bankruptcies and collapses after monetary union this summer, he said. "Nobody there knows how to sell. It's an alien concept."

So the Western companies wait.

"GM and the others are not going to do anything for the moment," Walter said. "They've already calculated that East Germans will earn Western European incomes by 1995."