If the lights are dim, the ground is bad. Either the wire or the connection (don't know where it is on that car) have high resistance.
The cruise control amplifier grounds through the brake light filaments, so that when you step on the brake and the brake lights come on, you apply 12V to the ground and the amplifer switches off. If you have a bad ground on the tailamp assembly, you get significant voltage on the brake light filament from the marker lamp being on, with a bad ground, so the CC amp is switched off.
Check the ground with an ohmeter, I'll bet you find significant resistance on the ground side of the bulbs.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!