While most big-car dealers are crying the blues, Tom Nelson is having little difficulty selling his gas guzzlers, which average 12 miles to the gallon.

His secret? Custom-made Cadillacs with built-in life insurance: blast-resistant, explosion-proof, $100,000 armored vehicles for customers more concerned with dodging bullets than topping tanks.

"People who are worried about their hindquarters don't care about gas, as Alexander Haig found out the other day," said Nelson, president of Odin International armored car company in Alexandria.

Gen. Haig, the Nato commander, was the target of an apparent assassination attempt near NATO headquaters in Belgium last Monday. A bomb blast threw Haig's chauffeured Mercedes 600 sedan into the air, but Haig and the driver were uninjured.

Immediately after the attempt on Haig's life, Nelson said he contacted NATO with an offer to sell them an armored car -- which Haig's was not

The company, one of about a half dozen such firms across the United States, is headquatered in an unmarked brick warehouse in Alexandria -- strategically located across the Potomic from Washington, which salesmen say is the hub of the armored car business.

Odin International recently sold 10 armored sedans to the State Department and distributes its glossy sixpage brochure to embassies, government agencies and private corporations around the world.

A State Department spokesman said the agency purchases armored vehicles from several firms, among them Odin International and Hess and Eisenhart of Cincinnati. "SURVIVE!" the Odin brochure claims. "Nightmare in the real world! "Kidnappers, assassins strike as you ride . . .open to attack."

According to the company, 60 percent of all terrorist attacks or kidnappings have involved an automobile. "There are a lot of wackos out there," said Nelson. "And these cars have saved a lot of lives." He declined to name any of his clients.

Hess and Esenhart president Jim Cetner said his customers have included former Canadian prime minister Pierre Trudeau, the Vatican and the queen of England.The company's armored Cadillac limos can cost up to $200,000 with extras like track spitters, grenade launchers, bomb-scanning devices and James Bond-like ejection seats.

Odin International offers a line of nine different custom-made cars from $70,000 to $100,000 with two levels of protection. Basic level No. 1 is advertised as protecting against all pistols and submachine guns and standard issue hand grenades.Maximum level No. 2 protects against assault rifles, light machine guns small explosive charges.

Both protection levels includes an explosion-proof fuel tank, blast-resistant floor, and reinforced front bumpers for ramming. Options include tire safety rollers [$2,593] in case of tire blowouts "due to gune fire or other reasons," sirens, red lights and loud hailer [825] and Halon Fire Suppression System [$1,383].

Discreetly located flag mounts and security buttons are included, free of charge.

The 360-degree fortress protection adds from 800 to 1,500 pounds to the car, according to Nelson. Customers may order armored Mercedes, Cadillacs, Oldmobile 88s, Chrysler New Yorkers, lincolns, Dodge Rams or armored Jeeps and vans.

"You really can't tell them apart from ordinary cars," said Nelson.

"And riding in one is a unique situation. It's like being in a cocoon."

Salesman Jack Kemp said, "If you're in the business, you can tell" an armored car by the chrome around the windows, the smaller rear window and the size of the doors, which are twice as thick as ordinary car doors.

"Plus," he said, "you cannot open an armored car door from the outside. It's permanently locked from the inside."

Odin International often sells to foreign countries, mostly the Middle East and South America, Nelson said.

"The brochure is very effective," he said. "You don't have to speak English to understand it."

But Nelson said he does not like revolutions or coup d'etats. "They'er bad for business," he said. "It's a gigantic foul-up. Everything stops for six months to a year." The company does not advertise behind the Iron Curtain. "We'er not interested in protecting Russians," he said.

Another consideration, according to armored car aficionado is the desination of the vehicle.

"They use different weapons in the Philippines than in Italy or San Salvador," said Nelson. "A car must be protected according to its geographical location."

Are any of his customers skeptical about his product?

I once had an Arab customer who said to me 'How do I know it works?" Nelson said. "He wanted to test it. So I told him I would sit in the car and he could fire at it. He bought the car on my word."

According to Nelson, "He was a lesser Arab. I told him if it was good enough for Begin and Sadat, it was good enough for him." CAPTION: Illustration, Armored cars manufactured by an Alexandria firm emphasize all around protection against acts of terrorism.