I have a breadboarded version of my GM E and C cassette emulator running in my Camaro. Here are all my design files. Consider them free as in beer open source, NOT FOR COMMERCIAL USE without my expressed written permission.
Grab the eagle file to see how I was interfacing with the E&C and the AVRs.
Use them at your own risk as always. Your mileage may vary.
Arduino Casette emulator and E&C Scanner
Cemu Attiny 85 Source code (GCC Compatible)
Eagle Files for Beta Aux in board - Eagle and Schematic
Collection of E&C Logs and messages
I will have a spare board or two if your interested in trying this in you older GM automobile. Drop me a line.