I tested the switch with a battery and my little probe. The switch works fine. I guess I'm looking for some reason why it won't work when I mount it on the post and push the brake pedal.

 

I should clarify that question a bit.

The master cylinder push rod is not pushing hard enough on the switch. The pedal moves toward the front of the car and the brake pedal arm pin moves within the push rod eye. I felt the spring compress a bit when I held my finger on it while pushing the brake pedal.

Turning the just made the brake more sensitive.

I can disconnect the switch from the push rod and compress the spring with a channel lock pliers and the brake lights come on.

 

Been a long time since I changed one of those but, are you sure you are mounting it correctly?

 

I believe I'm mounting the tail light switch properly. I've got a diagram from a shop manual. I also watched carefully when I took it off. It was working a while ago so I don't understand how the mounting could go bad to begin with.

 

Interesting. The original shop manual is wrong. I have a CD copy of a shop manual see http://www.FordManuals.com the 1966 Comet, Falcon, Fairlane, Mustang Shop Manual. This is a scan of the Ford Form 7760-66. Figure 5 on page 15-11 shows the push rod from the master cylinder mounted on the passenger's side of the brake pedal arm. It is mounted on the driver's side.

 

Oh, the diagram in the shop manual is upside-down. Just the diagram. All of the text labels are rightside-up.

My next step will be to bypass the little clip that connects the wires to the brake light switch. The clip is missing one of the parts that hold it against the switch. I tried holding the clip against the switch while pushing the brake pedal and testing the results. Just too difficult to do while lying under the dash.

I'll use a vampire connector to split both electrical leads.

The clip is part of the $430 wiring assembly.