The surefire way to get the cruise control going is to remove the amp with its clamp from under the dash. It is an aluminum box about 6 X 4 inches and there are a couple of plugs to remove. It will be easier to remove the bracket (10mm nut) than to remove the amp from the bracket.
You could try cleaning the cable contacts with a pencil eraser and/or tuner cleaner (don't smoke if you use it), and then snapping them back together. Maybe then it will work.
If it does, fine. If not, then check the connectors on the actuattor, which is a thingie about the side of a fist with a dome bolted on top of it. If this works, hooray.
If it still doesn't work, I would remove the amp and the actuator and send them to a rebuilder, such as Peter at GDI. Thomaspin has another repairer that might charge less.
The good news is that this will almost certainly fix it for years to come. The bad news is that it will cost from $200 to $400.
It is a dead accurate cruise control, unlike the sporadic things GMC puts out.
I personally thin it is worth it, especially if you take a lot of long trips.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"