If you can taste "something" in the water after letting the bottle sit in your car, then it's logical to conclude that "something" that wasn't originally in the water is leaching from the plastic. With #1 PET bottles, the main suspect, from what I have read in the past, is phtalates, although that may or may not be what is imparting any taste to the water.

Most plastics that bottled water come in are not as durable as I would like. The soft, milky LDPE plastic jugs are notorious for springing leaks while being stored in a car. I haven't yet had a PET bottle leak yet, but unless it was a thicker-walled PET container and not the thin, crunchy kind of bottle, I wouldn't really consider storing them in a hot car for months at a time.