Here are the steps I followed to install a VY Dome lamp in my VT II Berlina. This is the basic install process. There are also some separate pages on creating the template for the roof hole and more detailed information on the electrical aspects.
I started off with a small simple cut in an area I knew would be well within the final
hole size and near where I expected to find the mount holes.
Once I found the mount holes, I used a compass from the 2 mount holes to mark out
a center line (remember bisecting a line from primary school maths ?). You should
be able to see the intersecting arcs above and below the hole in the pictures below.
Then I positioned the base only template a bit higher (further from the windscreen) than I knew it would finally go and traced it onto the liner. Then I cut the hole with a sharp knife (I actually used a scalpel).
From this point you can also see that my bar only has holes for the two self tappers, but no rectangular slot for the locating lug on the top of the lamp, so I needed to remove the lug. I used a chisel, followed by cleaning up with a sharp knife. Also, removing the lamp section from the base will make things easier for the next step.
Next, I fitted the base-only section of the lamp into the hole, and held it snug against the forward most edge of the hole. Then I put a scribe through the screw holes and marked the bar.
You can see from this photo (the crosses) that I was 20mm high and a little to the right.
With this new alignment information, I could now mount the full template. I found that using some modified paper clips held the template in place without crushing the liner. You will also notice that I haven't followed the center line, it turned out that my sunroof switch is not central, so I decided to line up to that instead.
Once the full size hole was traced and cut out, a few trial fits showed that the rubber feet on the lamp section didn't touch the roof, and the unit was wanting to sit higher than the sunroof switch. I suspect this is only an issue with a sunroof fitted. To measure the gap:
Another critical step is to measure what size self tapper will fit, using a vernier caliper with a hole depth guage I determined that from the hole to the roof skin was 15mm. So it was safe to use 10mm screws.
With it fitted in place, the drivers side sits snugly against the lamp, but the left hand side does not fit correctly.
I checked against the template again, and the problem became obvious. So a quick trim of the corner and is was sitting well.
Now to wire it up. I started at the driver's B-pillar. The first step is to remove the B-pillar covers. The bottom half removes by pulling just below it's top edge. This reveals 2 screws at the base of the top section. Undo those and the single screw at the top (under a cap) to remove the top section. Just hook the seat belt around the head rest of the driver's seat to keep it out of the way.
Once removed I found this. I expect that the wires would normally go forward, following the black plastic guide. However as I have a sunroof the wires are routed slightly differently, to the rear of the car.
This is how the B-pillar
harness (92104481) will route.
I stripped the tape and plastic guide from the new harness as the guide in the new harness is identical to the existing one.
To ease access to pass the wires through the roof, remove the front visor and it's support. This should allow you to pull down the front edge of the lining. Also you need to remove the A-pillar cover. Just remove the over head handle and pull on the cover (starting near the B pillar) to pop the various clips.
Another trick is to glue the connector between the 2 harnesses to the roof so it won't rattle. I then taped the new harness to the plastic guide, and wrapped it around the old harness, taping at approx 4in intervals.
Finally I spliced the wires. At this point there are only 5 wires (orange/black, violet, yellow/blue, white/green and black). And for a Berlina/Calais you simply match wire for wire by colour. For other models and wiring options you may want to look at the electrical page.