I've fitted a dash cam in my car which also has a reversing camera feature built in before the start of our Australian summer. It mounts directly over the rear view mirror so it looks discrete. Many times during summer I've left the car parked early morning in direct sunlight and return towards the end of the day and as of right now there hasn't been any issues and it's almost the end of summer here. The only problem is having black leather seats in the sunshine.

I picked up a dashcam for the car black-friday last year, have only used it a couple times though.
I personally won't leave it stuck to the windshield in plain view (for both the hot sun & they say don't leave any electronics visible...)
When not in use I keep it in the house.

I also have a dashcam- just a cheap one.  runs that same software that all cheap cameras run- you know the kind.  sits in the car all the time and i havent had any problems with it.

Yeah I think mine has generic firmware or something. It has an AV input that the reverse camera plugs into. The reverse camera is wired directly across one of the reverse bulbs and the dash cam display automatically changes over to reverse camera when the car is put into reverse.

The dash cam unit also has a standard USB connection which shows up as Mass Storage device on the computer (32GB MicroSD) so no need to install drivers on the computer.

Something else I would be concerned about are possible phosphors used in a LED backlight for the display.

The Led's won't suffer at this temp, but the LCD screen might black out, however the LCD will return to normal as it cools down, my voltmeter does this all the time, but that's what happens when you leave it on the dash in the sun on a hot day..   

Is it safe to leave my camera in a hot car for a couple of hours, in full sun?
What about just lenses?

If you don't mind somebody smashing the window and stealing it, go right ahead 

The real problem is that the battery of the camera will catch fire or explode in a sunny day and the car closed at 50 degrees Celsius. Also, the heat will soften the plastic lenses and the video will look blurry, also the sensor will melt if you let it focused to the sun.

I have heard that dash cams use super capacitors instead of batteries due to them not exploding in extreme heat, but I can't confirm this.

Not heard that one before. I don't have a car at the moment, but when I did, the dash cam I used had a charger that you connected with the same USB cable that was used for downloading the video. So I assume it was a battery.

I have heard that dash cams use super capacitors instead of batteries due to them not exploding in extreme heat, but I can't confirm this.

They use small pouch style LiIon batteries. Their small size and so rather low energy content is what makes them rather safe. And maybe many camera makers do not charge them fully (above 4V for a standard cobalt based chemistry; instead of the typical 4.2V), greatly reducing the degradation, mainly at the high temperatures. It means losing about 20% capacity, but that is not a problem, since these batteries are needed only to properly close and finish writing the files on the memory card, so for few seconds after power is cut.
And LiIon is way cheaper than a corresponding supercapacitor plus its energy management circuits (wide range operating DCDCs). And the supercapacitor has its life limited too...