First, I want to say that I apologize for not having pictures, but I'm doing this from memory. I'm on vacation, and my car is not with me. That said...

Materials needed:
New Mini dome light assembly (approx $80 fro the dealer)
10' of RED 18GA wire, 10' of BLACK 18GA wire. ($4 from O'Reilly auto parts)
2 18GA 3M wire taps ($4, O'Reilly
1 XActo knife
1 Sheet of white, thin cardboard (art supply store)
1 pencil or pen
1 piece of thin, stiff plastic (I used part of the lid of a cat-litter bucket)
1 tube of GOOP (hardware store or Home Depot)
2 Spring-type clothes pins
1 flexible spring type "gripper" 2-3' long
Soldering iron
Solder
The procedure will reqire that you let the car sit overnight (for glue to dry).

I wanted to get more light into the rear of my car. The light that went on when I lifted the hatch lit up the floor of the area, but not much else. Worse, when I put something back there, like grocery bags, the light was covered and became unusable.
I wanted to add more light, but I also wanted not to destroy the "clean" look of the interior.
I called my local dealer and found they had a complete replacement dome lamp assembly (about $80). While I felt that was expensive, I figured that trying to find a suitable substitute would "burn" more than $80 of my time, so I bought the light.
I went to a local Aaron Brothers art store and bought some thin white cardboard and a new X-Acto knife.
I removed the "normal" dome lamp by sliding a kitchen table knife under one end of the oval lamp. When I slid it under one end, it hit a hard stop, the other had a bit of "give". I pressed against the side that allowed movement, and pulled the lamp down on that end to remove the dome lamp assembly.
I disconnected the connector and put the assembly down. I cut a piece of the white cardboard just slightly wider than the opening (in the small dimension) and about 2/3 of the length of the opening (in the long dimension) of the oval. I put the cardboard piece up through the opening, and holding it with one hand, traced the outline of the opening with a pencil. The cardboard wasn't large enough to trace the entire opening (since I had to hold it at one end with my hand), but large enough to trace a little more than half of it. Since the opening is symmetrical, that was good enough.
I took the cardboard out from above the "ceiling" and carefully cut along the traced line.
I laid the template I just made on another piece of the cardboard, and traced that outline. I turned the template around in order to trace the complete oval. Just to make certain that everything was aligned, I measured the traced oval (in the long dimension) against the opening in the car headliner to make certain they were the same. The end result was a (supposedly) perfect replica of the headliner cutout drawn on the cardboard.

I cut out a rectangle in the cardboard about 2" larger than the oval in each dimension such that the oval was in the center of the rectangle.
I then used a sharp X-Acto knife to cut the oval. Now I had a template that had an opening that matched what I needed in the headliner.
I used the new lamp assembly to test the opening in the cardboard to insure it was correctly sized. I also held it up to the original headliner cutout. Maybe I'm paranoid, but I didn't want to (eventually) cut my headliner incorrectly.

I put the cardboard up to the headliner in the place I wanted to put the new light, and held it in place with 4 sewing needles driven first through the cardboard, and then through the headliner. I measured everything to make certain it was perfectly centered and not "tilted". I also wanted to make certain that there was enough clearance above the headliner for the lamp, so I put tape on a sewing needle representing the height of the new lamp assembly near its periphery and shoved it through the headliner in 4 places around the oval. If the needle didn't hit anything, then I knew I had enough clearance. The new lamp assembly was "taller" in the middle, so I moved the tape and pushed the needle through the headliner near the middle of the oval to make certain that there was enough clearance there as well. I figured that the needles were a good way to check clearance, since they didn't leave any (very) visible holes. If they hit something, I would have to move my proposed location, but fortunately, I didn't have to do that.

I used a ballpoint pen to trace the opening onto the headliner and removed the cardboard template (which had been held in place with the needles).
I then used the X-Acto knife to "saw" through the headliner. It is a bit difficult, since the headliner material is fairly soft, and has a foam core, but with a little care, the cut oval came out cleanly. I did notice that there was a wire bundle on the left side of the cutout (when viewed toward the rear of the car from the inside), so be a bit careful about that when you cut.

I took the red and black wires and twisted them together (I used an electric drill, but it can be done by hand).
Next, I removed the left access cover (when viewed from the rear) in the 'boot' area. If you reach up, you can feel the backside of the boot lamp. There is a clip on one side. If you depress this clip, the lamp assembly comes out easily. Once the lamp assembly is dangling inside the car, you can use the quick-connects to connect the wires.

You may want to remove fuse #16 (if I remember correctly) before you do this. This shuts off power to the circuit you are working on. This is a 10A fuse, and is accessible under the cover in the right foot well.
Carefully pull the rubber weatherstripping off the left rear roof pillar so you can see what is under there.

After the wires are connected, use the long gripper to help thread the wires up the left rear roof pillar. BE VERY CAREFUL! THERE IS AN AIR BAG IN THAT PILLAR! Thread the wires up the pillar behind the air bag. Run the wires very close to the weatherstripping. Continue pushing the wires up until then get to the opening in the headliner and pull them out into the interior of the car.

Next, cut two pieces of stiff heavy plastic (approx .030" thick) about 1" X 2" in size. I used part of a lid from a plastic bucket that previously held cat litter. Put a good bead of GOOP on one side of each piece and place them on the far side (top side) of the headliner on both ends of the cut out oval, with the glue on the side facing the headliner. These extra pieces of plastic are needed since the headliner is too "flimsy" to hold the clips on the ends of the lamp assembly without some reinforcement. Use the two clothespins to hold the plastic pieces in place until the glue dries. This will take 4-5 hours.
GOOP is a bit smelly, so you will probably want to leave the windows open during this process.

I didn't have the proper connector for the new dome lamp assembly, so I used the XActo knive to trim some plastic off a small portion of the dome lamp. There are two metal strips used to conduct electricity from the lamp socket to the connector. I carefully trimmed away some of the plastic to give me access to the metal strips, and then soldered the wires directly to those strips. After I was done, I covered the connections with a thin layer of GOOP to insulate them.

I pushed the lamp assembly up into the headliner, pushed the rubber weatherstripping back into place, closed the left 'trunk' cover, re-installed the fuse and was done.

Total cost - Approx $100
Time - 2 hours (not including glue drying time).