The same solar radiation that makes South Florida the ideal place to get a tan also helps make it the worst place on the continent for a car body, says the head of General Motors Corp.'s environmental testing grounds here.

That's why GM picked this site, about 20 miles southwest of Miami. to test whether its current and future products can stand up in the worst of all possible weather.

GM figures if its cars' paints still will shine, and their upholsteries don't crack here, then they will stand up to any climate anywhere.

Mango trees and paint plates share this 10-acre tract that engineer William Slater admits is in the boonies. Silence is in the air; but there is something else in the air that GM is more interested in.

"Humidity," said Slater, a pleasant, unrushed man who has worked for the company here for more than 28 years.

"There are three factors that help break down materials," he said. "They are solar radiation, temperature and humidity. The Gulf Stream is right off the coast, and its wind pumps humidity into the air while also stabilizing temperatures year 'round."

The third factor, the Forida sun, is legend.

Take away any one of the three ingredients and the aging process stops. At the GM testing plant in Mesa, Ariz., where the temperature hits the 110s, "Paints look like brand new after a year's exposure -- as compared to here," Slater said.

The reason is simple; the air is dry.

"Humidity keeps a certain amount of water on the car," Slater said. "And H2O is part of the oxidation process."

In the Midatlantic region, the three factors are present for only a couple months during the year.

"In general, a car's paint will fail three times faster here as up north," he said.

Some colors stand up better to this subtropical climate than others. Naturally, they are the lighter, non-metallic and pastel shades that reflect the heat rather than absorb it.

In Washington, a tourist distinguishes himself by the way he dresses. In Forida, a tourist distinguishes himself by his car's black interior.

The outdoor test lab looks fairly unscientific. Rows of black wooden boxes raised from the ground hold hundreds of 4- by 6-inch paint panels. Some panels are up to three years old; others won't be ready for judging until the 1983 models come out.

About an inch of each paint sample is covered for later comparison.

Paints from GM's three major suppliers, Du Pont, Pittsburgh Paint and Inmont are measured objectively for gloss, bleaching, discolorization, haze and fading. The vast majority of samples hold up well, but there are always some that don't, Slater said.

"Here's one," he said, pointing at a burnt orange paint the sales department named Saffron Metallic. "You can see where a haze has developed after three years."

A nearby red metallic panel did the opposite of blushing after two and a half years: It turned steel gray.

Lab personnel also scratch some of the plates to see which paints best withstand rust.