Originally Posted by 97 SL320

Are you using a digital volt meter? Unless you use a stray voltage eliminator, a digital meter will produce false readings in this environment making diagnosis difficult. Using an incandescent test light will produce better results as it loads the circuit.

A digital meter can help when used like this. With the meter on DC volts ( to read up to 15 ) connect one lead to the lighter socket ground ( the outer ring ) and the other lead to a good body ground. ( a bolt that leads directly to the body ) When things are turned on, the meter should read sub 0.5 V . Connect a analog meter or incandescent bulb to the positive of the lighter ( center ) and body ground.

If the car is losing connection from the ground side of the lighter to body, the meter will read 12V or so. If the positive is losing connection, the lamp will go dim / out.

The failure point is going to come down to a common power feed / ground point. We need a power distribution / ground wiring diagram to go any further.