Just been out in the car and back!
I had parked my car in the sun, but had the windows down a bit so it wasn't baking when I got in. Today's temp is about 29C with a slight breeze.
The unit was 33C when I set off. I had the aircon flat out and it took about 15 mins to really cool down. The player gets no direct air, (my next project). it stayed at 33 for 5 mins and then rose to 34C and stayed there for the remainder of the 35 min drive. I parked the car in the sun again but with the windows up and left the unit in the car. When I came back about 10mins late, the temp was 39C but it stayed there for the rest of the drive.
When I pipe some aircon over the unit, I would expect that to drop significantly.

My dash setup has a Alpine CD player at the top, a 12mm gap filled by a piece of aluminium angle to hold the alpine and empeg docking cages; and the empeg cage below it. There is a bit of air between the two units to allow circulation.

Overall I'm quite happy with the results, I can't hear the fan too much inside the dash.

What resistor should I put in to reduce the voltage to 8 or 6 volts?