I am going to be doing some work on my truck and want to know where the most rear point on the truck there is switched 12 volts and where there is brake only 12 volts (not brake and turn signals).

 

In both cases, switched 12 Vdc and brake only, I suspect that the best place is under the dash. Wire sizes are now sized closer to actual need, in some cases smaller than in days of old. All the extra wiring is dedicated, I'ld be hesitant to draw from a circuit that powers the FPDM for instance.

Much better to use the accessory feeds provided in the cab or run your own fused circuit with relay switched by acc terminals.

With the brake only hot, I don't believe that there is one after the line from brake light switch to turn signal "multi function" switch. What we call "brake lights" are actually fed by "signal wires" from the turn signal (multi function now a days) switch and they are usedv to feed the lights steady for brakes until the driver signals left or right in which case that side is disconnected from the brake light feed and fed through a circuit fed by a flasher. Even on my '60s cars, the brake switch worked the brake lights, but the line from the switch was tied into the turn signal switch which itself alocates power to brake lights dependent on position as chosen by the operator.

You could run a 12 volt hot with brake circuit like above, but with the relay switched by the brake light switch (pink / lt green) or the feed to the high mount brake light which is the only brake only light (it doesn't signal turns) I can think of.

 

I believe there is a "brake light" only wire at the rear bumper. (I know for sure my 2000 F250 has one) The single wire is intended to be used for the center brake light on a topper shell. If I recall correctly, the wire is light green and has a plastic sleeve covering the end.

 

Check your trailer plug wires, if you have a tow package