Thanks to information I had found on this forum I fixed my cruise control amp today! I took it out and resoldered every single solder point, put it back in and it works! The CC had never worked as long as I've owned this car so I'm really happy now.
The only problem is that the cruise operation is unsmooth. I mean everything works fine, it basically holds the correct speed, cuts off while braking, resumes when I push the lever and everything but in order to hold the speed the CC continously engages and disengages for a second or so which causes an unsmooth, uncomfortable ride.
what should I do next? speedo cable? speed sensor? throttle lube?
or is it the amp and I have to resolder again?

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1987 300E 185k miles