Did you check the switch? I recently went through the same thing, however I did not try the resolder as they won't accept the core for rebuilding if you do. If you checked the switch signal in addition to everything else, you probably need an amp. I sent my amp and actuator to GDL and it's on the way back as I type. My actuator was OK, but I sent it just to be sure because a faulty one will blow an amp. You may try a salvage yard as you just might get lucky. Look for low milegae cars. I couldn't find any 14 pin amps but I found a dozen 10 pin. That's my luck. Even if you find an amp that doesn't work, you could use it as a core for a rebuild. If you find one in a 'pull your own parts' kind of place, refer to it as a 'cruise control switch relay' - do not use the word computer or amp (save $$) And you never know, you might get lucky and find one that works.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick