I've been busy installing cruise into my '84 745i. The unit I'm installing came out of an '86 so it's probably similar to your unit. First, I thought the cruise is in the driver speaker slot but I'm sure Jens knows better. Second, It sounds to me like there might be a bad ground that is jiggling as you drive. The cruise takes input from a speedometer sensor, from the neutral switch light and from your brake switch. It also takes ground directly. If you are game to look for a bad ground I'm happy to fax you the diagrams I've been using. Just drop me an email with your contact information.

Good luck!

-Ted R.
'84 745i "Fraulein"

Here's what's happening: The cruise motor starts making all kinds of clicking noises in rapid succession when I turn the key - car doesn't have to be running, but it also does it when running. When I apply brake, it stops. Let off brake, clicking again. I'm sure it's the cruise motor because the cable that holds the throttle is moving a little when it's making the noise.
What's really going on here, and how do I fix? Is it getting some false signal to engage cruise control? Sometimes, infrequently, the car behaves. I had a 633 that did the same thing one week, and magically it stopped, but I don't think I'll be as lucky twice.

Thanks for your help everyone,
James