Well, just confirming you tested the wire that went to the engage terminal right? If so there could be a couple things.
Before I get into the wiring, if you have some extra wire and clamps or an extra set of hands run a wire from a good ground or the negative battery terminal to the case on the transducer. Then run the other wire from the positive battery terminal to the engage terminal on the transducer and see if it clunks. (Don’t let the wire from the positive battery terminal touch anything else while it’s hooked up to the battery or you might get a spark and smoke show lol.) If it does go “clunk” you know the transducer solenoid is engaging at least. Not out of the water yet but it’s a start. When you’re done with that, before you move on to below, also take your multimeter and set it to measure resistance (ohms). Touch one lead to a known good ground or the negative terminal on the battery and touch the other end to the case of the transducer to make sure the transducer is grounding okay. If you get a reading on your meter move on to below. If it reads zero you might have two problems at once.
If the transducer clunks with the test above and it’s grounding fine try the following
#1, there could be a break in the wire somewhere but that’s probably not likely so we’ll skip that for now.
#2, look under your dash for a small black connector. It should have some pretty tiny wires coming out of one side that are stuck together so to speak. It makes them look like one flat and wide wire but if you look close theres actually 3. Assuming the colors are the same for 1980 they should be blue black and brown. Make sure that didn’t come unplugged somehow. That’s the plug to your cruise button on the turn signal stalk. I can’t post pictures here but I can try to find you a picture and maybe email it to you if need be.
#2.5, there may be a fuse around there that’s blown. It would be an in-line fuse and not part of the fuse block. I say “may be” because there is a reference to a fuse in the manual and I’ve heard other people mention it but my 77 does not have a fuse that I’ve found nor does an extra Wiring harness I have.
#3, if that’s plugged in and the fuse is good (or not there) then I’d move on to the brake pedal switch. The switch on the pedal doubles to activate the brake lights when you press the pedal and it also breaks the circuit feeding power to the cruise so when the cruise is engaged it disengages when you press the brakes. That switch could be out of adjustment (though not likely if your brake lights work okay) or maybe the plug for the cruise cane off for some reason.
Let me know.