Plastic access cover? Hmmm... All I have on the headliner is a dome light. I'll take a look at the service manual tonight and see if it offers any clues about the wiring in the ceiling.
My other car is a Miata, and the wiring harnesses in those are the same regardless of what options are present. So I have the wiring for power windows/locks/antenna, cruise control, even though I don't have those options. If I wanted to add them I'd just have to install the hardware and plug it into the existing wiring. In general, is that true of Jeeps too?

1986 Isuzu Trooper, 2.3l, 119k miles when acquired 3/1996, 231k at time of death 9/21/03.
1995 Jeep Cherokee Sport 2-door 4x4, 4.0l, 5-speed, 85k miles when acquired 11/2003, 138k now.