Never mind.
I didn't understand the markings on the fuse panel. Now that I have a fuse box diagram my voltage drop isn't as bad as I thought.
Thanks for the help.

Power = current x voltage. So you're telling me that circuit is drawing 18.3 amps & not blowing any fuses. This assumes a full 55 W for both bulbs or 110 W total. Hmmm.

I guess 6 volts is okay if your fuse hangs in there.

BTW, there's always a voltage drop as the load increases to FEWER ohms. Zero ohms is an infinite load with no voltage across the load or what is now a short.

I think that's an 8 amp fuse so the circuit would be limited to 8 x 13.8 = 110.4 watts
At 6 volts I suppose it is less than 48 watts total to be consumed by the bulbs.
I would assume there is resistance upstream.

Check all of the connections at the battery first.
There's usually a bunch of them.
Verify good grounds to both engine and chassis.

This should always be done first IMO tracing electrical gremlins on these cars.

I had non-working condenser fan on a customer car recently.
All the stuff up at the battery was NEW.
One was bad and came apart as soon as I tugged on it.
Don't assume that they are good visually.
Take them apart and inspect/clean.

It WILL piss you off to do XX hours of diag and it come back to basic power supply.
I know from personal experience!

To what ground are you comparing the positive voltage?

I'm with the Ninja, sounds like grounds.

The door hinge.

There seems to be a good chassis ground and ground to the fuse panel The three or four wires coming off of the positive post all had low resistance. The unswitched fuse terminals did not drop in voltage when I attached a 55 what bulb to their circuit.