I'll start this one off by answering the question: Hot enough to melt plastic. I know this from personal experience.

(And by the way, if you have your own sob story or stories about how you had an item melt in the car, feel free to post it as a comment.)

The first time I had something melt in a car from Summer sun was when I was still living in New England and I left a ZZ Top audiocassette (remember those?) on the dashboard. In August. It had been left in the car the previous Friday night. Saturday came and I didn't have to leave the house until around 2pm. I get in the car, which at the time was an '84 Honda Civic, look on the dashboard and my tape is warped. Completely cooked from the sun.

I then felt rumbling pangs of regret.

Darn. I really liked that tape.

The second time I had something get cooked was years later when I was using a Garmin StreetPilot c340. The mounting bracket was in such a way where the power connector plugged directly into the mount rather than the GPS itself. So what I used to do is pop off the StreetPilot but stupidly left the cord plugged in and completely exposed to the sun. The rubber didn't melt, but it got hot enough to separate the inside wires just enough where if it were in the sun too long it just didn't work.

I felt those rumbling pangs of regret once again.

Stupid, Richard. Stupid. You know better.

I live in Florida now where it's a lot hotter, so I have to be more mindful of what the sun can do to my tech stuff.

This is what I do, and I suggest you do the same:

Don't use the glove box to put electronic stuff into

The glove box acts like an oven when it heats up. You're better off putting the whatever-it-is on the center hump or under the seat where there's more air and cooler temperatures.

If it has a screen, always place it screen down

Example: A point-and-shoot digital camera. If you're going somewhere to take photos and have the camera on the passenger seat, have it screen down. The lens is covered and protected, but the screen is not.

If it has a soft carry case available for it, get one

Soft meaning not leather. While it's true leather does a fine job protecting your stuff, you can grab a soft shell case easier as it won't burn your hand.

Check your exposed devices on long trips

If you have to drive more than a half-hour somewhere, check to make sure whatever is exposed to the sun (GPS, cell phone, etc) doesn't feel too hot to the touch. If it does it needs to be brought out of the sun.

Don't use the A/C to cool off an electronic device that's too hot

This is when you pull the whatever-it-is off the mount and hold it in front of a vent with the A/C on to cool it down. Bad idea. Could cause condensation. Moisture and electricity don't mix.

Workaround: Have your fans blowing on cool without the A/C on. The temperature will be cool enough to cool the item down, but not so cold it causes condensation.

Did I miss anything?

What do you do to keep your electronic stuff cool (or cool enough) in a car cooked by hot Summer sun?