there are a couple of wires in the harness itself that bolts onto the computer that have to be switched and one i think that should be sodered.. but other than that - all you need is the computer itself. I wouldn't buy a used one, just find a crap one, even a speed density one, i have a couple extra around and some other guys do too, and take it to autozone, tell 'em that your 89 computer went to crap and you want a new one... brand new, like 90 bucks
you'll have to get with someone else on how and what wires to cross.. it's only 4 wires to mess with though, and not that expensive.