My 68 Cuda, converted to front discs and redid all brakes, then off for FE alignment, the best it had drove ever but my Speedo quit on the way home, had to retighten the lines into the new prop valve and got the speedo cable back into the dash, now the interior lights come on dim when brakes are applied, done for the day!, hopefully figure that out tomorrow. Any thoughts.

Try checking ALL your fuses to see if the are ALL good. Everyone of them

Check grounds for all lights.

Grounds and then bulbs themselves. I had an internal short in one that turned on my park light. Would be weird to do an inside light.

Had a customer years ago with the same complaint. Turned out to be a blown brake light fuse.

My guess might be the brake lights have a flaky ground and are getting their ground through the dome lamp circuit which also includes the glove box and trunk lights.
What happens with the 4 way flashers or turn signals?

These members have some good suggestions. I too think it's a ground problem. But until you figure it out, maybe put a red translucent cover over the interior dome light and have the first ('68) car with a third/high mount brake light. Sorry just joking. Maybe pull one brake light bulb out at a time and check the result?

If you have recently worked on the left door sill or the left kick panel remove the screws again and check if that solves the issue? DAMHIK. I did that with my '68 RR and inadvertently ran a screw through a couple of wires.

Thanks for the humour, I needed that, fix 1 thing break another over and over again last 3 weeks, hopefully I pinched an obvious wire today. I hope! Dave

The interior lights light because the door switches ground the lights or the headlight switch grounds the lights. I'd look at the headlight switch.

Check your grounds especially at the tail brake sockets. Migt try a jumper from say the bumper or clean metal spot to the socket housing

This is what it turned out to be, my wiring harness looks suspect, better start saving now. Thanks for the help guys. Dave

glad it was a simple, and super inexpensive, repair.
however, if the harness looks suspect, replacement sooner rather than later, is the way to go.

I have had same problem floating on quite often, and was a brake lights fuse related problem, but not because blowing… some bad contact at fuse clips. This bad contact was able to heat enough to melt the fusebox

it is kind of amazing just how much heat can be caused by a bad contact/resistance in a simple 12 volt system !

I even got my finger burned when I touched it!

About the heat… think on the cigar lighter!

Bulbs itself! Headlights! Everything of that are heating elements on 12 volts circuits.

Never would have guessed that.