Dave Holls, a ruddy-faced, General Motors design executive, does his best middle-aged imitation of a young boy in a candy shop. He peeks out of the double doors of the top secret Cadillac styling studio, crooks his finger, smiles impishly and beckons a visitor into forbidden territory.

Inside - where outsiders are almost never allowed to venture - the air is thick with the smell of clay. Clay models are the stuff of dreams in the auto industry, and this room is where GM is dreaming of a brighter future.Just past the doors, up on a raised platform in the expansive, flood-lit studio that recalls nothing so much as an artist's loft, sits a full scale prototype of the 1992 Cadillac Seville. It is the embodiment of a design revolution now under way at GM.

Rumors have been making the rounds in Detroit in recent months about the revolutionary design of this car - about how this car represents GM's most complete break yet from the hidebound, look-alike designs that have plagued the world's largest automaker. The rumors are true.

The long, sleekly sculpted body, in gun-metal stainless steel with advanced, wet-looking mica paint, shows hardly a trace of Cadillacs past.

"I'm not supposed to be showing you that," Holls sighs, but he continues the tour anyway. He styled Big Cadillacs as a young designers back in the 1950s, so this is like home for him, now one of GM's top design executives.

Across the hall, on yet another platform, sits the 1992 Cadillac Eldorado, again the subject of rumor and speculation. It is far bigger than the current slow-selling model, with 3 inches more knee room planned for the rear seat. Much like the '92 Seville, the Eldo has sharply edged linear styling. Longer, the Eldo returns the car nearly to the length of its long-lost rear-wheel-drive ancestor, phased out in the early 1980s by a fuel-conscious GM.

Neither car looks much like anything on the road today - they borrow nothing from the Ford Taurus aerodynamic look that now dominates so much of the world's automotive design. And that's just the way Holls likes it.

"We don't want to be bland," insists Holls, who is mindful of just how much blandness has cost GM in the recent past.

"I look at the new Infinity and Lexus, (Japanese luxury cars) and I can't see anything wrong with them, but I can't find anything great about them either; they just have kind of bland designs. We don't want to do that at GM.

"When you see a 1989 Cadillac with those big tail lights sticking out to there," says Holls, knifing his hands through the air for emphasis, "you're going to say that's a Cadillac.

"We can't be bland, because we have five divisions and we have to make them look different. That's our approach."

For GM, such thinking about design represents a revolution, a dramatic break from the disastrous sameness of the recent past. GM says it has had a bellyful of criticism about its cookie-cutter cars and is determined to make it easier for people to tell its car lines apart. The change seems genuine, and GM is taking the unprecedented step of throwing open its design studios to a few outsiders - without cameras - to prove the point.

The first signs of the revolution are visible now on the street, in smooth lines of the well-received Oldsmobile Cutlass Supreme, part of the company's new "GM 10" line of intermediate-sized cars. While the GM-10 line suffers from poor interior design, its exterior styling marks a watershed in GM's new drive to be different.

But the cars that represent GM's real break with stodgy design have yet to be driven. Next fall, GM's startlingly futuristic all-plastic minivans will hit the streets; with their broad use of flushed glass and low, car-like lines. These vans to be sold by Chevrolet, Pontiac and Oldsmobile, should - along with similar offerings coming from the Japanese - redefine and shake up the minivan market.

Other cars still squirreled away in the design center, and shown only to a few outsiders, would help end the criticism of GM's look-alike image. Among the most significant are thought to be the new Chevy Caprice, due in the spring of 1990; and two Buicks due in 1991: the Park Avenue with Jaguar looks and a longer Riviera. With these cars, along with the new Cadillacs, GM is clearly striving to differentiate its cars from each other.

But it has taken GM a long time to respond, and these cars still won't be on the road for a year or more. In the meantime, GM must continue to muddle through with its current lineup, paying for the mistakes of the 1980s.

Stuck with dozens of bland, look-alike models that have turned off buyers, GM's sales and market share have plunged throughout the mid and late 1980s. Customers no longer able to tell the difference between Buicks and Cadillacs turned to distinctive Fords and imports instead.

Top GM designers, gathered here for lunch recently to talk about the problems of their recent past and their optimism about the future, recall one dark moment when the look-alike problem really hit home to them. In 1983, Fortune Magazine ran a story on GM's cookie cutter image - complete with a cover photo of several GM sedans lined up side-by-side to show how difficult it was to tell them apart.

But such criticism has been just one of several key factors that have helped GM design launch its recovery.

More important, designers say, a new sense of freedom has been flowing in GM's design center since the widely respected Chuck Jordan took over as GM's chief of design from the methodical Irv Rybicki, who retired in 1986.

Jordan has insisted on greater creative freedom for his designers and has made sure they have the time and resources to express themselves. He has restructured the design operations in GM's 36 studios to free up designers from many administrative and budgetary responsibilities. New business staffs have been set up to take on those chores, so the designers can spend more time doing what they do best - designing cars.

Surprisingly, designers also say that GM Chairman Roger Smith, heavily criticized in the press for GM's look-alike woes, has in fact been pushing for more avant garde styling in recent years. He has encouraged the new sense of creativity fostered by Jordan.

Now the results of the new sense of freedom and creativity are parked all over the design center; prototype cars covered with tarps to keep out prying eyes line the corridors and sit atop pedestals in modeling studios. Only a few outsiders have seen them, yet these new designs have impressed observers, who see a total break with what GM now has on the road.