OK, I'm having a brain fart here. Of course electrical is my weakest (except for bodywork) automotive issue.
 
After nearly getting rear-ended on SEVERAL occasions, I ordered a LED billet third mount brake light. Rather not have it, but it's better than a totalled Rambler.
 
I wired it in the brake light circuit. It's grounded to a good body ground and the positive is in a pig-tail adaptor in the left side brake light.
 
Prior to this install, brake lights worked well. The brake light switch in 1965 was a single-wire pressure switch in the hydraulics. Has a newer switch installed.
 
After the installation, the third mount brake light didn't light up (it was bench tested prior to install). However, if I unhooked the plug from the light socket, the third brake light WOULD light up with the brake light wire still connected to it. If I reinstall the plug the new light goes out but the regular brake light goes on OR blows the fuse. New fuse and the factory lights work fine again, but no third brake light.
 
Made me very curious. I hooked up a test light (battery in trunk) to the positive post, and probed the spade on the brake light socket. It lit up, indicating a ground. Huh?? Even with the harness hooked up to it, a probe indicated ground. BUT, when the brake pedal is activated it goes hot lighting the brake lights.
 
Might be worth noting that the brake light circuit requres the ingnition switch to be on. I checked the wiring diagram and that is correct.
 
I am confus-ED.

Yeah, we figgered it out on the other forum. It has to be BEFORE the column. So I have to run a wire from the front of the car (switch) to the back of the car (third light)
 
It's not just the jerks. When I'm driving east late in the afternoon, the sun shines into my taillight lenses and it's hard to see the brake lights. I may even hook up a flasher on the third mount.

The better "third brake light" kits are a three wire type with a logic chip. You have a ground, left hot, and right hot. The logic chip then can "watch" the brake lamp circuit on both sides of the car and keep the "third brake light" on even when you have a turn signal going. It also will flash the third lamp if you turn on the hazard/4 way flashers. Best part is you tie it all together at the back of the car and don't have to run any additional wires from the front. Since you already have the third light you may want to check to see if the vendor offers the logic chip as an additional purchase. Just a thought.
 
Ken

I replaced my 1157's with LED's in my '66 Classic Wagon and they are better.  Needed to upgrade the flasher unit though.  Thanks for the info as I plan on getting one of these CHMSL -industry jargon- lights (Center High Mount Stop Lamp).