I did this the day after I got my '89, because the tumbler was all messed up. I could turn the key to on, and even start it, but it would take 10+ minutes of fiddling with it to turn it off again. I had blisters just from turning the damn thing off.
It's fairly easy to pull off. I didn't have an FSM at that point, just a trusty digital multimeter. Basically I pulled the panel off of the steering column and pulled the connector off of the ignition switch, then used the audible tone on the ohm meter to figure out which pins on the ignition switch were connected in OFF, ACC, ON, and START. Then I spliced onto those wires and ran them up where the radio should have been, and wired it up to a cheap autozone ignition switch (which has four poles on it - battery, accessory, ignition, and start). Later on I added separate switches and a push button, so it could be a whole big ordeal whenever I needed to start it
The wires going to my ignition switch were:
- Red - supplies constant +12V from battery
- Brown - receives power when key is in ON position, but NOT when starting
- Orange - same as brown
- Yellow - receives power when key is in ON position *and* START position
- Green - only receives power when key is in START position
I made the assumption that brown and orange were accessories (ever notice how your accessories go away when you're cranking), yellow was ignition (needs constant power even when cranking) and green was start (obviously only getting juice when cranking). Verified it later against the FSM once I got one.
These were all moderately sized wires, 12-14 ga or so. There were also two smaller wires (18-20 ga) that were purple and gray, which get shorted to each other while the key is in the START position. They are on a separate connector and I didn't bother hooking them up. No problems so far, but again it might be worth checking an FSM to be sure. The wire I spliced in to run to my switch panel was a bit smaller, only 16 ga probably, but as far as I can tell the wires don't get warm. They're not sourcing much current as relays or solenoids are used to actually engage things like the fuel pump, starter, etc.
I am in the process of going back to a single ignition switch, which I will probably mount where the factory clock is. This keeps it in a standard location and will work pretty well I think. Note that I didn't have to wire into anything under the hood or snake wires all over the place or run thick wire directly to the starter, or anything crazy like that. Good luck!