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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Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.