Exactly, because the corrosion we both agree on has introduced a large voltage drop. The regular bulbs take a lot more current, so the drop will be greater, and volts will read really low with a regular bulb, maybe only fractions of a volt.

 

So should I go straight to the header? or try pulling the light out of its spot and look at the wiring behind it? Tonight I was going to try running a wire for a new ground to the light as Savannah said. Also, I got your picture, is that up under the glove compartment or on the right side of the passenger's feet?

 

Grounds problems are very common, try the ground wire first. If that has no effect, backtrack the 12 volt wire. If it helps, backtrack the ground.

 

the pix says rear, so I take that to be the door area (belt tensioner), and luggage area. It may be easier to simply take a small piece or wire and attach to a known ground (like a metal bolt into metal) and touch other end at socket; for testing.

 

Well I ran a wire from the negative battery post to the interior light above to the dash. No change so it must be coming from the hot side. I am thinking it must be around the switch to the doors since the lights do not initiate when the doors open unless its drawing the current down too low because all of them are trying to turn on. May get a chance track some of it tomorrow.

 

Door switch does not carry the lamp current like in the old days. It drives the BCU, which then handles that function.