WHO says cars are bad for the environment?In California, if you buy a new Geo, the company will plant a tree in your name. "The program has gotten us a lot of media attention," says Jeff Hurlbert, the general marketing manager for Chevrolet, which distributes the Geo line of small cars and trucks. And these days, Mr. Hurlbert says, the name of the game is "getting closer and closer to the consumer. The person who doesn't play in that game probably won't survive." There are some fundamental changes under way in the very nature of marketing cars. Until recently, most manufacturers divided up the bulk of their advertising budgets between network TV and major national magazines. A small portion went to local print or broadcast, most as so-called "co-op" advertising aimed at supporting local dealers. Today, however, that approach is becoming less and less effective - and for a good reason. It's no longer the Big Three lording over a bunch of small imports. Today, 40 or so manufacturers compete with lavish ad budgets adding up to several billion dollars a year. A prime-time network movie or weekend sports event may feature a half-dozen carmakers all vying for the attention of the consumer. "It's difficult to get your commercial to stand out amidst all the clutter," says Ray Serafin, who covers automotive marketing for Advertising Age. Making matters worse, while network ad rates have skyrocketed, "the networks don't deliver the audience they used to," he says. So to increase their overall reach - and to target just the most likely buyers - the carmakers have been rewriting the rules of advertising. In California, Chevy's best-seller is the sporty Camaro. In Chicago, it's the subcompact Cavalier. So Chevrolet spends a third of its budget on regional ads focusing on products local consumers are most likely to buy. In the effort to catch consumer attention, carmakers have come to realize that even with regional marketing, they may still have trouble standing above the clutter. So many are exploring new concepts of "unconventional" marketing. In recent weeks, Chrysler Corporation has mailed a quarter-million videotapes to owners of the company's Voyager and Caravan minivans. It also sent another 100,000 videos to owners of competitive products, including General Motors' APV minivans. GM's Pontiac division - which markets the TransSport APV minivan - quickly fired back its own salvo. It shipped out its own six-minute videos to 255,000 Chrysler minivan owners. But Pontiac's package also included a $500 rebate coupon for anyone trading a Chrysler minivan in on a new TransSport. VIDEOTAPES have become the hot new ad phenomenon, thanks largely to new cassette designs that can cut costs down to as little as $1.50 a tape. "Even though people know it's a commercial message, the number of people who actually sit down and watch a video that comes in the mail is staggering," and probably runs at least 70 to 80 percent, says John Damoose, Chrysler's vice president of marketing. Tapes have a distinct advantage over more conventional print brochures, adds Ann Pollack, who handles marketing for Toyota's Lexus luxury division: "If I can bring the car to you live and close, rather than static on a piece of paper, it will deliver more of that excitement." Lexus offered videotapes as a come-on to potential buyers of the new SC400 sports coupe introduced earlier this year. In order to get a copy of the tape, buyers had to call a special toll-free number and provide some extensive personal information. The program generated the names of 35,000 qualified buyers, "a pretty aggressive number," says Ms. Pollack, "considering the overall luxury coupe market is just 120,000 a year." GM's Buick division has experimented with a variety of unconventional marketing approaches, including regional advertising, videotapes, and even floppy disk for home computers. These contain product-line information, prices - and games. For 1992 models, Buick expects to mail out 150,000 floppy-disk catalogues. "On the average, a customer will spend 20 minutes with the disk and go back to it as many as four times," says Nancy Newell, Buick's electronic marketing director.