The reason I ask about the cruise there is a clutch (or that is what we will call it) in the cruise actuator that gets its power from the cruise control unit and it's ground through the brake light bulbs. If the resistance through the brake lights is to high, the clutch won't enguage and the cruise dosen't work. When you hit the brakes, then the actuator doesn't see a ground anymore, it sees power and the cruise goes off.
Now back to our problem. I believe there is a bad ground somewhere associated with the right tail lamp assembly. The way to prove this is have someone turn the ign on and press the brake pedle. Look very carefully at the right and left sides and compare the brightness of the bulbs. If the right is dimmer then there is a ground problem. Now turn on the parking lights with the brakes lights still on. Check again at the brightness of the right and left, this time also looking at the brake lights and the parking lights. If something is different then there is probably a bad ground somewhere. Continue with these test using the turn signals and backup lights. With the answers to the above test answered, we can narrow down the problem.
It takes much longer to discribe the test than it does to do them.
The two most common problems we would see was either the ground between the bulb and the bulb socket or a bad connection somewhere in the tail light assemble. The above test would lead us to the problem 95% of the time. To actually pinpoint the problem we used a $10 dollar test light, not a high dollar or a high tech piece of equipment.

PaulM

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84 500 SEL (307,xxx miles)