I had a '81 Silverado with cruise. I think they were about the same truck '81-'88. My cruise control unit was on the driver's side mounted on above the wheel well. I believe it had vacuum hoses attached and of course an electrical plug. It it has been removed, you should have some empty mounting holes in that area. Good luck.

 

Take it to the local Chevy or GMC dealer, and simple ask the service manager or service writer if you can ask one of the techs a question. They will usually accommodate you, (unless they're complete butt heads).

I took my 2000 Silverado to the Chevy dealer about 3 years ago, and had the broken tail gate cables replaced. Turns out there was a recall, so the cables and the labor were free.

Sometimes it just pays to ask.

You'll also find that many of these techs do side work at a far lower cost than the dealership charges for their talents. Just make sure you tlk to someone who knows there stuff, and not some back yard Bill who does electrical connection with tape.

 

Ran into a fella driving a cruise equipped 84' and he let me take a looksy under the hood.
Looks like (in 84 anyway) the cruise equipment was mounted behind the carb, on the manifold, close to the firewall, and slightly offset to the drivers side.
this included some electricals, throttle cable, and what looked to be a small vacuum housing of some sort which makes sense since these are the basics to a crusie control unit.
Compared, I'm outta luck. No cruise here dang it.