Years ago, I installed an aftermarket cruise control on a 1979 Malibu station wagon, then a number of years later another aftermarket cruise control on a 1989 Ranger, 2.3l with a 5 speed. In both cases, none of the wiring had anything to do with the factory wiring, unless you count finding an unused key turned on circuit in the fuse box. When I plugged a spade connector into the fuse box, I used an inline fuse. It took a few hours the first time, but a lot less time the second time. All that was needed was to run their wire harness, find a vacuum source that had a big enough hose to provide the suction, mount the cruise control unit in the engine compartment, attach magnets to the driveshaft and a sensor close enough to read them. It was easy enough that I'll likely put one on my '90 model at some point in time.