Ideally, if you can do this: (may be hard, you need to get a serial cable to the empeg when it's in car)

stty raw -echo 19200
od -txC -v -w4 /dev/ttyS0

...and press buttons, you should get a line of output for each buttonpress (and release). Annotate the text with the labelling of buttons on your dash and we can look at it. v1.03 does have very narrow ranges for buttonpresses, v1.04 will fix this but the data from the process above would be useful.

Hugo