I wouldn't do any repairs yet, or even replace the fuses yet. There's a dozen good places to find the defect, and one of them could even be where the 12-volt-feed fuse box terminal has the fuse snapped into it. Continuing on with my previous procedure, you would touch the meter's or test light's probe to the fuse box terminal, then to the end of the fuse plugged into it. Those two points are shown with the arrows in the first photo. If you find 12 volts on the fuse box terminal, (blue arrow), but 5.4 volts on the fuse, (green arrow), that connection has to be where the high resistance is. Look at the rust on the terminal just above the green arrow. That is a dandy place to suspect a high-resistance connection.
If you were to pop the correct fuse in first without doing any testing, the scratching action is going to scrape off enough of any light film of corrosion that the high resistance is gone. The circuit will work now and you'll never know why it didn't work before, and you'll always be worried it's going to act up again.
Electrical specialists including myself often complain about Chrysler's wiring diagrams that bounce around between multiple pages in the service manual, and each page has just a tiny portion of the circuit on it. Even back in the '90s, there could be over 100 pages of diagrams to cover just one or two car models. But here is the exact opposite. The entire electrical system for a '72 Dodge Challenger fits neatly onto two pages and is easy to follow. GM's engineers thought it would be easier to put everything on one page, and here's what they ended up with in the second diagram. It is completely impossible to know where each circuit runs because they don't show which switch wires are connected when turned on, or which terminals in the fuse box have a fuse between them. This is just too impossible to follow, however, coupled with my wondrous memory, we can still figure out where to go.
My memory of wire colors in the '70s still holds true for a '66 model. The brake light switch is shown at the far left of this diagram, and the orange wire is the 12-volt feed. If you follow that wire to the right and down, you'll see it comes from the fuse box, but I also noticed it runs to the head light switch first. That tells me that circuit feeds something more than just the brake lights. That has to be the tail lights and / or the head lights. If we knew which other lights that feeds, and if those lights work correctly, we would know everything, including everything inside the fuse box has to be okay. On the other hand, if the brake lights are dead, and the circuit at the head light switch is also dead, we would know the defect has to be before the head light switch.
As a side note, if that orange wire at the head light switch is feeding the head lights along with the brake lights, a 15-amp fuse is not big enough for that circuit. One high-beam head light draws close to six amps. Two would draw a total of 12 amps. There needs to be some safety margin built in to prevent nuisance blown fuses. They normally don't want to exceed 80 percent of a fuse's rating under normal circuit operation. Add in any tail lights that might be on that same fuse, plus the intermittent brake or signal lights, and even a 20-amp might be too small. Once we get this solved, we can worry about figuring out which are the correct fuse sizes.
The first step is to measure the voltages in the fuse box, but I'm going to jump ahead since you asked about the switches. The third photo shows what your brake light switch looks like. The arm of the brake pedal presses on the white button at the top left to turn the switch off when that pedal is released. Either terminal can have the orange wire plugged into it, then the white wire will plug into the other terminal. The switch mounts to a metal bracket that may be attached to the steering column or to the frame of the dash board. The terminals will be open and exposed on the back where the wires go in. Those are easy places to take voltage readings.
The white wire runs to the signal switch. It sits right under the steering wheel, but testing it gets to be real easy. The fourth and fifth photos show two views of this switch. The switch contacts are on two halves that are riveted together and are non-repairable. One half is stationary and mounts to the steering column. The other half rotates, or slides over the stationary half to make different connections for the turn signals. As such, the turn signal lever is bolted to that movable half. You can see in the fifth photo the wires run into the assembly, and there's no place to take voltage readings. You have it much easier. The fourth photo shows the two connectors, and you'll see that long wire harness they're attached to. Those run through the steering column and come out under the dash board.
The sixth photo shows one of those connectors, and the seventh one shows one connector turned just enough that we can see where the wires run in to the terminals. That's where you can poke the probe to take the voltage readings.
When you apply the brakes, the brake light switch switches on the 12 volts that goes to the signal switch. With the signal switch released, current flows through it and out on two different circuits, one for the left brake light and one for the right brake light. If the defect is before the signal switch, both brake lights will be dead.
What many people forget or overlook is the brake light switch only supplies the 12volts for the brake light function. You still need 12 volts to the signal switch for the turn signals when the brake light switch is off. That means there has to be a second 12-volt source to the signal switch. That one comes through the flasher and it is likely to be on its own fuse. We know that because the brake light circuit can operate any time, but the ignition switch has to be turned on for the signals to work. The point of this part of the story is the two rear circuits might work properly when the signal switch is turned on, but not when the brake light switch is turned on. There is value in knowing that. If the signals work in the rear, that is proof those bulbs, and the wiring all the way back to the signal switch, including at least part of the signal switch and some of the connector terminals have to be okay, and there's no need to do any testing in those areas.
Images (Click to enlarge)
Saturday, March 13th, 2021 AT 5:09 PM