I had a 90 legacy that didn't have cruise. i went to Harry's and got all the parts (throttle thing, computer, and 2 switches) and just plugged them right in. The car is wired (the 90 was) already and it was pretty much just plug and play.
I would say...maybe pull the thing out of the dash where the activator switch goes and see if there are any wires there. That should answer your question as to how hard this is going to be.
The only thing that is going to suck is you have to take the airbag out to put the steering control switch in.
