I've thought of several ways to address the issue:
- change the 5V rail to a switching design
- put bigger resistors and mount the 7805 on a heatsink
- have a switching or linear preregulator that drops 12V to a more manageable 7-9 Volts
The bigger resistor solution would also work but I did not have any 10W resistors at hand and I'm not sure they would fit in the small space between the CD tray and the case.
Since I have a few LM7808 and LM7809 from scrap I decided to go with the third solution. So I would basically bypass both resistors, go through the 7808 to drop 2-7V and use the 7805 to drop the remaining 3V.
I started by removing the "leaking" resistors and dead regulator. See the black stuff left behind by the droppers?
The LM7808 is prepared by soldering some heavy gauge wire and add heat-shrink tubing for insulation.
See part one here: http://hackcorrelation.blogspot.com/2013/09/teardown-and-modding-car-mp3-player.html
The "new" LM7805 is put into place, soldered with an underpowered iron and has the input leg lifted. The input will be tied to the 7808's output, meaning the third pin.
Below the 5V regulator you can see the small 3 pin LDOs (lower left), AMS1117 with the top one providing 1.8V (memory?) and the bottom one 3.3V (for the microprocessors).
With the power resistors removed from the circuit the connection is performed via a wire on the backside.
The regulator grounds are tied together, the output from the 7808 goes to the input of the 7805 and the input of the 7808 is taken from the old input of the 7805.
The 7808 is screwed to the case and soldered before the final turn so that it can have a good thermal transfer. It will have to dissipate almost double the power than the 7805.
There is an important omission to my changes: no buffer/stabilizing capacitor close to the input of the 7808. This can cause it to oscillate in some situations but I have not seen any problems in my testing.
I left the rig running for a few hours while charging a phone and having the supply at 15.5V. The screw got really hot (70-80C) but it stabilized at 70C. I think before this modifications the resistors were running at well above 100C.
In the end I am happy with the result, I can finally charge my phone or the GPS without using an extra car charger socket and it's at least ok for emergency situations when the charger breaks down (happened twice) or you really need a free 12V socket for other stuff.
I wouldn't charge a tablet on it though, I don't think the solution is able to supply more than 1A continuously, even that.