Many thanks,I have a multimeter,but am sorry to say,do not understand it.I have a simple light up screwdriver with an earth wire attatched.my car is a 1990 super sprint . the last car I touched was a sprite in 1970 due to company cars . the black loose wie gets lost in the loom .if I conect the 2 wires ,the blue live screen wire with the dead black loose car wire can I try it without sparks or damage,. The light on the screen switch works ok .again thanks