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I noticed tonight that my taillights were very dim compared to normal. After a little bit of troubleshooting, I noticed that my taillights are good as long as my instrument/map light switch is in the off position. If I switch that to either the instrument light or map light position, the taillights get noticeable dimmer. There does not seem to be any impact to the headlights brightness.

Looking at the wiring diagram that I put together some time ago in conjunction with the original Packard diagrams, the problem is not obvious. Has anyone else experienced this? My only thought right now is that it may be a bad/dirty ground connection, but I’m not sure where.

Thoughts?

I'd get out a multi-meter and see if/what the voltage drop is taillights with the maplight on/off.

There is a possibility that there is some kind of minor short it that map light switch.

If you see a drop when the swtich is in either position, then try removing the switch from the dash and let it dangle so it's no longer grounded, and see if the problem persists.

If it suddenly goes away, then the switch has some kind or minor internal short.

Power follows the easiest path. So a minor short in the swtich and/or combined with dirty grounds or dirty connections somewhere between headlight switch and the taillight sockets could cause the issue.