(FORTUNE Magazine) – AT THE SPECIALTY EQUIPMENT Market Association trade show in Las Vegas--the giant annual wingding for people in the business of customizing cars --there is a rule of thumb, with a corollary. The rule: You can tell how well an exhibitor is doing by the number of scantily clad models it can afford to hire. The corollary: Hiring too many models can be a mistake. As Dean Pugh, who is manning the gleaming, chrome-filled booth for Monét Wheels, puts it, "Sexy models are essential in this business, but there's a fine line. You don't want the girls to be so hot that no one looks at the stuff you came here to sell."

Heads are turning up and down the aisles at this year's show, and it's not just because of the short skirts. Diamond-studded lugnuts, 24-speaker stereo systems, nitrous-oxide injectors, underbody neon lighting, pneumatically adjustable suspensions, flashing tire-valve caps--the bling goes on for 12 linear miles of exhibits. SEMA has become the second-biggest trade show in the country, after the Consumer Electronics Show, and possibly the most important car show after Detroit. "We take it very seriously," says Ed Golden, Ford's director of design. Why? "We want them to be tuning up a Ford."

As the business of aftermarket parts has exploded --sales are expected to surpass $29 billion this year, more than twice what they were a decade ago--it has become something Detroit can't ignore. That's why Golden and fellow Ford designers Larry Erickson and Patrick Schiavone are here, their suits and gold name badges making them look a little out of place among the "tuners" in T-shirts and oil-stained jeans. "The thing is, today everybody's reliable," says Erickson, who designed the 2005 Mustang. "Everybody's quick. Everybody's got quality. So now the personality thing becomes more and more important."

That can mean anything from flashy chrome wheels to $10,000 entertainment systems with ten flat-screen TVs to paint that changes color when viewed from different angles. It started in the 1990s when California teens--much like their parents in the hot-rodding 1960s--started turning beaten-up sedans into street racers. Their racetracks were long blocks and riverbeds, some of them the same places where earlier generations competed in souped-up Mercurys and Thunderbirds. Only now technology allowed hot rodders to get high performance out of smaller, four-cylinder cars like Honda Civics. Some swapped out the computer chips governing engine functions; others lowered suspensions for better aerodynamics. Some just added bass speakers in the trunk or chopped off mufflers to add "noise." But now, a decade later, "tuning" has moved to the exclusive brands. At the SEMA show, there are Porsches with doors that scissor open like those on Lamborghinis, Jaguars with racing-style suspension, and six-figure Bentleys with tinted windows and 20-inch wheels (called "dubs" on the street).

Ford gives top aftermarket designers new vehicles for $1 before they hit the dealerships so that body kits and exhaust systems can be ready when the cars hit the market. (The new Mustang has a customizable instrument cluster, in which six light-emitting diodes can provide up to 60 background colors.) The way Ford figures it, thousands of kids are getting hold of cars and either tricking them out for speed or customizing to some extent, usually a little of both. If Ford can get them hooked on its cars now, after the kids grow up, they'll still buy new Fords. And they'll customize them too. Ford estimates that Mustang owners spend some $800 million making their cars go faster and look flashier.

GM's idea is a little different. It wants the money at the front end. Interested in carbon-fiber engine covers and body-side molding for your new Cadillac STS? Want chrome exhaust tips and restyled grills on your Pontiac GTO? Your local dealer can hook you up. GM wants to rake in $1 billion in accessory sales by 2008, a quantum leap from the $200 million it earns today. "Last year we tried out this extremely tricked-out Chevy Aveo with bright-yellow panels" at the show, says Kip Wasenko, GM's special-vehicles design director. "This year your dealer can order that body kit for you." GM has also started introducing high-performance versions of its family cars, like the 275 horsepower Pontiac Bonneville GXP, which debuted at the show this year. For 2006 it'll even add a "Corvette-inspired" production Chevrolet TrailBlazer SS based on the concept shown in 2002. "When the folks around here like it, we use that as the justification to move forward," says Wasenko.

It was actually the Japanese car companies that first capitalized on the show. Two years ago Toyota introduced a new brand, the Scion, designed specifically with young tuners in mind. The base price is low--from $12,995 to $16,465--but the average buyer drops $1,000 more on factory customization such as colored steering wheels and pedals and accent lights below the dashboard--all highly profitable items. Toyota can't keep up with demand. Nissan, BMW, and Mercedes-Benz already offer larger wheels on some models, and the 2005 Volvo S50 has optional, semitransparent Iced-Aqua plastic trim that adorns the center console. Other automakers are pushing performance "sub-brands." The new MazdaSpeed Miata comes with a turbocharged, 178-horsepower engine (36 more horses than its tamer sibling), 17-inch wheels, and a lowered suspension. Even Rolls-Royce is at the show: Its $320,000 Phantom can be ordered with 21-inch aluminum wheels that "add a dramatic edge to the look of the motorcar."

But it's the Big Three that spend the most time and money here, and it's a chance for the guys in suits --all of them car nuts at heart--to get in touch with their inner Von Dutch. You have to be a little naughtier, a little raw, to succeed in the aftermarket business. Schiavone, who heads up Ford's truck design department, wears a positively beatific expression as he takes in one vehicle the company gave to a customizer with instructions to go wild. "There's no way I can come to any vice president at Ford Motor Co. and say we're going to do a flat-black truck with purple flames and a green pinstripe," he says. "But now we've got one on display on our stand."

The guys don't spend much time at their own company's exhibit. Their mission here is reconnaissance and inspiration. Skulking around the Nissan exhibit, Erickson pockets his Ford logo and asks one of the salesmen if they really are offering sandblasted wheels with the new Z. (The answer is yes.) "See, we should be doing that," he says. "If you grabbed our marketing folks, they'd say if you want to charge a premium on it, it's got to be chrome. But look around: painted finishes, coated wheels. It's all ideas." But what Golden and Schiavone notice is the three buxom women, in tall black boots and halter tops, who are signing autographs at the Toyo Tires booth. The line stretches 20 deep. "Look at all those guys with camera phones," Schiavone says. "Like they're actually taking a picture of those tires or that car. It's a shame really. Beautiful girls though ... wow. Really beautiful."

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