My 1972 Duster has an interesting problem that I can't quite figure out. The passenger side tail lights (running and stop light) are brighter than the driver side. I swapped bulbs side to side and that didn't change anything. I checked the wiring with a voltmeter and they both read right at 12 volts. Both sides are operating properly, one filament comes on with the lights and then the second one comes on with the brakes. So everything checks out but the right side is definitely brighter than the left side. I looked at the lens to see if one side looked different than the other and I can't see any damage. They both look the same but it is almost as if the left side doesn't allow as much light to shine thru for some reason. Both sides appear to be original to the car, I don't think either one is a reproduction since they both have the Mopar part numbers and they both have some wear and tear on them like you would expect from 50 year old parts. Kind of stumped, anyone have an idea?

Andy, try cleaning the bulb socket and the mating area on the body. What you are discribing is a loose ground and the bulb is finding it ground thru the right sides bulbs, thats why they are extra bright too.

You could also run something to touch to the assembly to ground it as a test to see if it's the ground. I did that to a few things before installing. I used auto wire with alligator clips on it. I also used that method to find a good ground for electric flashers that needed to be grounded to something. Easier because it gives you a length of wire to work with and you don't need to disassemble/assemble something all the way to get it to work.

A voltage check without a load on the circuit can be misleading. If you check voltage with bulb in and on, does it read 12V? Chances are no. If it does, that would point to the lens. The measurement has to be on the terminals at the bulb socket if you can get to it. Otherwise you need to rig a jumper from the socket.

So is the dim side correct or the bright side the goal? If you find the problem will they both be bright or the lesser?

I found in my car the previous owner coated and painted a lot of stuff. So the grounds were poor for the tail lights, since they go through the body.
They make new pigtails for the lights with an extra tab on them for a ground wire.
I did that and it eliminated the ground issues.

but as said above you might want to test that out.
add an extra wire around the socket where it "holds" on to the tail light housing and give it a solid ground see if the dim light comes up to the other side.

Did you check the flutes on the inside of the lenses for dirt/grime? I'm sure the lenses come apart, outer red and inner reflectors. Also, sometimes the reflectors are faded or gunged up as well.

Some good ideas in here. I have found socket corrosion/dirt to cause this issue in the past. Might also verify the socket wiring isn't reversed and illuminating the brake filament instead of the running element.

On my '69 B body I had that issue with the front parking lamps/turn signals.
I had to create some additional grounds to make the lights the same brightness.

JUst went through this on my RR. Ground. Like mentioned above....make some leads and test

My taillight harness is brand new so there is no corrosion anywhere. All connections are bright and shiny. I can check voltage drop with the lights again but so far I haven't noticed any difference from side to side.

OK, then like said above, use a true grounded wire and clamp it onto the bulb socket. If the bulb gets brighter it must be the housing not getting a good bite to tin. Or the housing is corrored where the bulb socket turns into.
If you were closer I could drop by. I'd give you a couple JP Popper clips with the Bed of Nails.

Fresh paint?

What I'd do is with a length of wire with an alligator clip on one end & a long thin pointer on the other end, ground the alligator clip to a KNOWN good ground & take the lens off & reach in with the pointer & touch the bulb metal case & see if it brightens

dave, where does one buy those "popper" clips ?
TIA !

There was a thread somewhere recently and his issue turned out to be the inner socket disc in his new harness had been installed 180 degrees off.

I saw that thread and I checked that. My bulbs are both operating correctly. Each side is on the correct element for run and brake. I also double checked the grounds and they are fine. Adding a second ground doesn't make any difference and the voltage drop to ground is very small. I'm pretty sure it is a problem with the lens.

Pull the sockets, run a jumper to ground if needed and compare bulb output. If they are different with the sockets out of the housing it;s the bulbs.

Do one more test then.
with or with our sunglasses, pull the bulbs out with the extra ground wire on both sides.
see if they are both normal brightness to you without the housing.