testing ground
 

The best way is with a volt meter. However, you can also check for ground with a 12v light bulb connected to 2 leads. Attach 1 lead to the '+' terminal on a socket (an aligator clip attached to the lead is handy for this) clip the other lead on to the taillight or front indicator light's metal housing (the part that s supposed ot be grounded). If you have a bad ground, the light will not light up. Move the ground wire around touching bare metal surfaces (screw heads and such) until you get a circuit.

One of my rear indicators had a broken ground wire (there is a ground wire in the wiring harness that is supposed to attach to the signal housing). Simply reattaching that wire made my dimming rear light problem dissapear. Even with a broken ground wire, things will sometimes work since the body is metal and there is usually some form of metal-to-metal contact to the signal housings. The problem is that these connections are subject to corrosion, moisture and even bumps in the road!

Don't ask me about the Lucas (prince of darkness) electrical system in my ex-MG!!!

Michael