Older cruise control systems got their throttle servo ground from the hot side of the brake light circuit. It's. Safety feature that makes sure it disengages when you hit the brakes. If your control box has failed, it could be putting a load to ground on the brake light circuit and dimming the lights. Disconnect the cruise control box and servo connectors and see if it helps with the lights. The older analog cruise controls were made by Dana, and I suspect you will be out of luck finding a new or rebuilt replacement for something that old.
Installed one on a 1970 GMC van conversion I built. Loved it when it worked right (most of the time) discovered burned out brake lights made it malfunction. Hence the troubleshooting ideas above.
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'The Silver HamShack' (2007 International 22FB CCD 75th Aniversary model)
Multiple Yaesu Radios inside and many antennae sprouting from roof, ProPride hitch
2012 shortbed crewcab 4x4 Toyota Taco TV with more antennae on it