Don't forget, early Mk2 models [below serial number 1000 or so], were missing the Earth wire on the harness serial port, which would mean no data would flow.

You can check this by testing when the empeg is docked that pin 5 on the serial lead is connected to the metal chassis of the car - use a multimeter with either ohms measurement or "continuity tester".

If its missing then you can either:

1. add the missing earth to your serial lead (somehow)
or
2. Add a earth wire from your Pic chips Ground connection to the Empegs metal cage or grounded surface on your car.