My ‘75 did not have a light to tell you the parking brake was on, but it does have a wire going to the proportioning valve, which turns on the light in the dash if there is a brake line failure. This light works by grounding the wire to turn on the dash light.
Newer trucks did have parking brake light switch (like shown above), so I looked up the part number and bought one. Took a little time to figure out how it mounted, but it bolted right onto my brake pedal mechanism.
I connected one side of the switch by splicing it into the existing wire going to the dash light (tan/black?), and for the other side I ran a wire to ground. It works great, and (ignition on) turns on the brake warning light once the pedal is down about two clicks.
