1. Is it harmful for GPS units to be left in cars during hot days of the summer?
I don't own a GPS unit, so I don't really know what's inside it. However, constant temperature change will eventually render any LCD display useless. If you live in an area that has a lot of humidity, that could also cause problems with the device. Condensation can build up inside the enclosure and cause pins to short out. You could probably prevent that problem by drilling holes in the enclosure (just make sure you take the circuit board OUT first if you plan to do this).
Yes, this is hack-a-fix 101
Try to avoid FM transmitters if possible. The sound quality will be downgraded severely(FM radio is far from cd quality).
The thing is, MP3s are a lossy format to begin with so an FM Transmitter isn't going to make it that much worse. If you want good sound quality, play CDs in a CD player.
I actually find that MP3s played through FM Transmitters sound BETTER than actual FM stations. Radio stations use a LOT of compression on the audio they output, and it ends up sounding like crap on the recieving end.
If you can get an FM transmitter with an actual antenna, I suggest going that route. The one I have has the antenna inside the cable that goes to the MP3 player, but a quick solder job will fix that
