I've got an '89 and Laybackman provided much info to me to get it wired. There should be a red plug under the dash to the right of the steering column near the heater controls connected to nothing; that will connect to the defroster switch and was part of the standard dash wiring harness. The wiring from it routes to a 3 wire plug above and to the left of the fuse box, also connected to nothing. Might be wrapped in foam and buried someplace up in there.
Check to see if there is a red relay socket near and above the emergency brake pedal; if it's there, that's for the 10 minute defroster timer relay and your jeep came from the factory with that option installed. If the red socket is missing (which is probably the case), you'll have to buy the entire harness, complete with socket and relay, over Ebay or from a junk yard as others said; I got mine from a yard for $50.
Regardless of the results, here's the diagram Laybackman provided:
If you're unsure of the wire colors, pull the relay and look very closely on the bottom next to the pins. It's hard to see, you'll need to have the light just right, but they are numbered 1 through 5. Those correspond to the small numbers on the above diagram; then you can trace the harness to the relay.
Thanks again Lbm!!