Everyone knows that soft drinks are one of the biggest moneymakers in QSR-land, but Pepsi and one of its Yum! Brands fast-food partners are hoping that the lure of exclusive flavors will help them rake in even more beverage bucks. The cola giant and Taco Bell have pooled resources to create a proprietary tropical lime Mountain Dew flavor to complement the chain’s Mexican-inspired food.

Two-thirds of Taco Bell’s business is currently generated at the drive-thru, but tacos and burritos are so messy to eat in cars that most customers have traditionally taken their nosh home and passed on ordering a beverage in favor of grabbing a free drink from their fridge. The partners hope that limiting Mountain Dew Baja Blast’s availability to Taco Bell fountain dispensers will create the kind of consumer excitement that’ll drive more drink sales.

They’ve certainly picked the right brand to tinker with, given that internal research shows the chain’s customers are already two times more likely to do the Dew than the average American.

Mountain Dew Baja Blast may be available at Taco Bell indefinitely, but Pepsi is imposing limited runs on two other special-flavored sodas on its 2004 books. Looking to build on the 10% sales growth it experienced by offering orange-flavored Mountain Dew LiveWire for just six months in 2003, Pepsi released Mountain Dew Pitch Black (in honor of Halloween) in August for a 10-week run at retail. And in time for Christmas, the company has cooked up Pepsi Holiday Spice, adding a touch of ginger and cinnamon to its traditional cola.

Given that these limited-time bevvies have such short lives, Pepsi doesn’t plan to support them with massive marketing campaigns. But an in-theater spot is building some buzz for Mountain Dew Pitch Black in 4,000 movie theaters leading up to Halloween. Otherwise, the company is relying on word-of-mouth, in-store displays and its package graphics to pique the interest of consumers, who may want to stock up before their favorite flavors are pulled.