how many people leave their empegs in the glove compartment when not in use? will it decrease the life of the hard drive from the heat inside the glove compartment?

If the temperature in the glove box exceeds the stated non-operating drive specs (also in the FAQ), then the answer is yes it will decrease the hard drive lifespan. I'm guessing that since you live in LA, if your car is parked in the sun, it could easily do this.

is the trunk cooler?

Dunno, I'm sure it depends on the car, but I know that my car's trunk is probably a little bit cooler than my glove box.

But the real problem is the fact that you're leaving it in the car at all. The trunk and the glove box are not safe from theft. Trust me, I speak from experience, having had two CD players stolen.

How much do you like your player? And are you aware of how difficult it would be to replace it if it were stolen?

BRING THE PLAYER WITH YOU. DO NOT LEAVE IT IN THE CAR AT ALL FOR ANY REASON.