Ok, I'm a car audio noobie and I've spent a reasonable amount of time with Search. I didn't see a consensus, so I'm going to ask with some specifics. What if I'm going to install a Alpine amp (70 RMS x 4) under my rear seat in a BMW 2002. There's free air space of at least 2 - 3" all around. Seeing as it's relatively low wattage compared to the bazillions of watts you guys run, will it be ok in that location? All advice and help is most appreciated. Thanks!

You should be fine with a 2002. Most BMW's made in the last 20 years have so little space between the seat and the floor you could not cram one in there let alone not expect it to overheat.