From memory, I removed the rear carpet metal trim from the back hatch, in order to pull open the lower rear plastic interior body panels, so that I could remove the upper two plastic panels at the rear of the truck. I removed the four grab handles, the four pieces of plastic behind the four upper seat belt mounts (have to unbolt the seat belt guides from the pillars first), the five (not sure on quantity) plastic buttons in the headliner, the two existing dome lamps, and lastly, the upper rubber trim that covers the raw edge of the headliner at the back of the hatch and above each of the rear passenger doorways.
After doing all this, the front of the headliner is still attached, which means the rear of the headliner needs to be supported at all times, otherwise it will break. It is made of thin brittle fiberglass. Best to have a helper. And also have on hand 3 or 4 large spring clamps. With some padding, the spring clamps can be used to clamp the rear of the headliner up, so your helper can rest her arms from time to time.
All of this is per my '97. I don't know how your '93 will be different from mine, other than you will have the wire harness for the cargo lamp, my '97 did not.
I did not install the smaller factory rear cargo dome lamp. I obtained a factory middle row dome lamp, milled a spacer from walnut, milled the lamp to mount flat on the spacer, all so I did not have to cut the headliner and the overhead sheet metal. All I had to do was make one very small hole in the headliner for the wire, and four small holes for the mounting screws. This approach was a lot more work, but avoided the risk of the "one shot only" of cutting the sheet metal in the right spot.
Given the approach I took, I don't have any information to direct you where to cut. I mounted my lamp horizontally centered. I don't know if the factory cargo dome mounts exactly centered or not.
