Is this a RioCar or Empeg?
If you "forget" to connect +12V power on the empeg, it will
actually run of the ignition-sense wire, and that will cause
this behaviour. Altrnatively, it could be that your Car sends
a volotage spike when you turn of the ignition. This can be fixed by instralling a diode, the empeg-guys will send you one for free if you ask.

In the case of the RioCar, both these issues are fixed.

Marius (Escort Cab + Mark II)