The factory Cruise Control electronics for 84-89 Vettes are contained in the Digital Cluster.
I'm hearing more reports of digital clusters not containing components for cruise control. This is because so many clusters are being swapped around to fix defective gauges that sometimes a car which originally shipped with cruise control has a cluster which doesn't have electronics to support the cruise function. There's really nothing to do except trade the cluster for one with cruise.
A user tip from Bob Golden:
"The cruise control driver transistor P/N was 16008121, a number I came all too familiar with. Almost every time you find the transistor open in the cruise circuit, you can bet the solenoid valve in the servo is burned. This holds true with almost all vacuum controlled GM cruise modules. If the level wiring shorts it usually just blows a fuse. We didn't see too many problems with the cruise circuits until these cars bean to age and the servos begin to intermittingly short. If you catch it in time you can save the circuit by changing the servo out. These servos are expensive but a lot less than a cluster repair. I never had much faith in the rebuilt servos, they tend to surge. You can find the new ones at a better price if you buy from a AC-Delco distributor. They range from $110 to $150. This is way over priced but that's how it is."
Click Here for a set of schematics on the Cruise Control board integrated into the Digital Cluster.
The most common reason that Cruise won't set is the TCC kickdown switch located above the brake pedal. It slides away from the brake pedal quite easily. The Cruise electronics think the brakes are applied and won't engage the cruise.
Click here for a list of known problems with the cruise control associated with the digital cluster.