Check the car's weatherstripping
You don't want to find out about leaks or cracks the hard way. Dampness and musty odors can signal leaks in the weatherstripping around the doors or windshield, says Brooks.
Brooks encourages potential buyers of winter beaters to look under the floor mats to feel the carpet and check for leaks, mainly on the passenger side floorboard. Also, try to test-drive the vehicle at highway speeds when excessive wind noise or drafts will make inadequate weatherstripping obvious.
Leaks can signal a problem with the heater core, which can involve significant labor charges, so it's probably a good idea to steer clear of any vehicles with these types of red flags.