BIG SKY, MONTANA — WILLIAM HEUGH is worried.Although pleased by a string of rave reviews of the new Pontiac Bonneville sedan, the marketing manager for the car remarks: "It's still hard to convince people GM [General Motors Corporation] can build a quality car." Making a debut earlier this month, the full-sized flagship of GM's Pontiac division has one of the most attractive exteriors to come out of the GM styling studios in recent years, and a well-thought-out interior that puts controls in easy reach and gauges within easy view. "It can't do anything but win," says auto analyst William Pochiluk of Autofacts Inc. But winning has been tough for GM lately; the nation's biggest carmaker last week announced a loss of $785 million for the second quarter of this year. Over the past decade, GM has alienated many of its core buyers through a series of poorly-engineered products and an unfortunate propensity for quality problems. As a result, GM's market share in the US has sunk to 35 percent. But among the baby boomers, the target market for the Bonneville, GM has trouble attracting even 1 in 4. Pontiac officials concede they will have an uphill battle with the skeptical baby boomers. Many of them "won't even look at" a domestic model, says John Middlebrook, Pontiac's general manager. To get them to at least take a peek, Pontiac will launch an aggressive marketing campaign taking some of its cues - such as targeted direct mailings - from Bonneville's import competition. There will be a lot for them to brag about. The Bonneville has been completely restyled for 1992, with a softer, more aerodynamic shape. Add a bigger engine, new safety features, and Pontiac planners expect the 1992 Bonneville to go up against such imports as Honda's Acura Legend, the BMW 325, the Volvo 740, and Toyota's Lexus ES250. The top-of-the line models, the SSE and SSEi, are loaded with goodies such as Traction Control and a passenger-side airbag. There is also a new, supercharged engine. Bonneville prices are expected to run from nearly $20,000 for the base model, the SE, to just under $30,000 for the SSEi. "Today, about 30 percent of our SSE volume comes from luxury import customers," Mr. Middlebrook says. "I expect that could get as high as 50 percent." The Bonneville isn't the only new Pontiac product that may live or die according to how well the division does at winning over new yuppie buyers. As with the Bonneville, the 1992 Grand Am will be marketed more aggressively to import-oriented buyers. The '92 update features an unusually aggressive - read risky - front-end styling. The car appears both bolder and more roomy than before. Grand Am buyers will have a choice of several engines. But the base model uses an outdated 3-speed automatic transmission, while virtually every competitor now offers an electronically controlled and more fuel-efficient 4-speed automatic. Still, the general consensus in the industry is that both the Grand Am and Bonneville could prove big hits. A first-quarter survey by Integrated Automotive Resources found 32 percent of import owners buying a new car were intending to trade in their old cars to buy domestic models. "It's a start," says Middlebrook. "If you don't get their attention, awareness or consideration, you don't get a shot at the customer." While Pontiac sales are down about 12 percent for the model-year so far, that is not much worse than the performance of the industry as a whole. Analyst Pochiluk predicts the division will fare much better in 1992, thanks to its new models. "They should catch the wave of recovery."