find the positive and negative posts. In my OHC I had a Red and a Green wire for positive and black for negative.
There are two types of map lights that ive seen in the OHC one has a cheap piece of metal that when you screw the light in it makes contact with. The other is a better made contact no pics but if you want some I can get them for you.
The more wiring harness you get the better, I pulled down the headliner of the exploder and the A-piller trim and cut the harness at the point it went under the dash gave me about 4 feet of wire to play with.
Make sure you get the bracket.
1999 4x4 XLT 4dr 3.0L
Explorer - OHC/Center Console