Hello All

I have a 1993 Scorpio 2.0 Auto. I noticed that the interior lights were no longer working, and the same with the central locking. One night when driving home (obviously with my lights on) I nuged the door lever with my elbow and the Central locking worked perfectly! as did the interior lights. I got home, switched the car lights off and both the central locking and interior lights no longer worked again. It seems that both these will only work when the lights are switched on!

I had a look through my Haynes manual and found that all three of these systems seem to share a common Earth connection, and so my first guess would be that this connection is corroded or something. the trouble is, I have no idea where I will find this earth connection to check.
Can anyone point me in the right direction, or give me any other advice on what I should check?

Thanks.