There are multiple thing that can make the Cruise inop, the stalk switches, the brake switches, the cruise module or the Servo.
You basically have to to test each system individually until you find the issue. With Cruise control, because the actions caused by the switches are too far away, its best to have a helper, with good ears.
First, verify the fuse is good. On some G-bodies there is a fuse spot that says "Cruise". I like to put the rear axle on jackstands so you can test it, when it gets to that point.
Turn the key to run and turn on/off the cruise button. Your assistant should hear clicking under the hood, when its turned on. If not, diagnose if power is getting to/from the stalk switches. There is usually a 4 wire connector under the steering column you can check for voltage changes when the CC is turned on/off.
If the stalk has power, Start the car turn the cruise on and set it and verify you have vacuum at the servo. If not, find out why not. If you have vacuum at the servo, look for tears in the rubber servo bellows. Pinch any tears closed with your finger and see if the cruise starts working.
The last thing to check is the CC brake switch, See if the continuity changes when the pedal is depressed.
The Cruise has a solid state module that could go bad, but what usually goes bad is something mechanical, made of plastic or rubber that goes bad or breaks.