I shielded the MP3 player board with a tinfoil box, didn't seem to make much difference in the range problem. That being said, I decided to go a different way and do it with TMCC anyway.
Here's the new interface, this will go between the ERR MiniCommander ACC board and the MP3 player module. The opto isolators are not technically correct as TinyCAD didn't have any AC opto component, but you can get the idea.
Basically, the four channels of the MiniCommander connect to the four optocouplers. Since two of them are negative in respect to ground, this seems to be the easy way, and it's 100% bulletproof. The output of the optocoupler is filtered by the 22uf cap to eliminate the AC ripple from the input side. The resultant signal is sent to the MP3 module to trigger one of four sound selections. The MP3 module takes a connection to ground (or at least a logic zero), so when the output from the Opto is active, that's what I get.
If the breadboard works as I expect, I will probably immortalize this in a PCB so I can install them in more than one car. Hand wiring a bunch of these ain't in the cards!
Edit: remove old schematic.
