PKR how easy was the rear interior light mod to install ?
It's not hugely difficult, but it'll be quite fiddley with the headlining in place.
Electrically, ( this is assuming post May 97 cars with N82 services unit* ), I picked up the Purple/White wire at the base of the 'A'pillar where it comes down from the front courtesy light and extended that back along the sill and up to the rear of the car under the parcel shelf. The Purple/White wire earths the permanently live light unit via the N82 services unit.
At a minimum you then take that wire, an earth wire ( you can earth it with the main earth near the battery ) and a +12v supply from the battery and connect to the 156 ( or whatever ) lamp unit.
What I did was a bit more involved. At the base of the 'A' pillar I took the wire through one of the G73 connectors and integrated into the car's wiring along the sill. I then installed a relay on the rear bracket and the wire from the front then only earths the coil of the relay. I took +12v from the battery and added a fuse, and that then goes to the other side of the relay coil and to the lamp units ( I also have rear footwell lights but they are still a work in progress ). The lamp unit return wires ( that would normally go to the N82 via Purple/White is then earthed by the relay. This means there is no appreciable additional current through the N82 unit. The additional earth from the lamp unit allows the light to be switched on manually.
In order to route the wiring you'll need to remove the sill cover ( easy ), the rear seats ( easy ), the rear parcel shelf ( a bit more involved ) and one of the 'C' pillar covers. Removing the parcel shelf is necessary in order to remove the 'C' pillar otherwise it could probably be done with the parcel shelf in place.
The wires go from the under parcel shelf area up the 'C' pillar and across to the lamp unit. As I had the headlining out for recovering, I was able to place the wires inside metal box sections fairly easily. With the headlining in place this will not be possible. If you leave the wires loose in the roof, I would recommend wrapping them in foam so they don't tap on the metal work!
This leaves the cutting of the hole for the lamp unit. As you can see in this photo
there is a substantial metal frame from the rear window over the rear headrests. You cannot put the lamp unit in that area as there is no room between the headlining and the metal. You'll see in that photo as well my wiring exiting the hole in that framing and passing in front of the rear window.
The distance on mine from the rearmost edge of the headlining to the closest edge of the lamp unit hole is 18cm ( best to double check with a needle before starting to cut the hole if the headlining is on the car! ). If the headlining is off the car, measure from the mounting holes that fasten the rear of the headlining to the car to the edge of the metal frame and transfer to the headlining.
The headlining board is not difficult to cut - in fact in some respects it is quite fragile. In view of this, I glued some steel on the inside ( roof side ) of the board to reinforce the areas where the lamp unit clips in. Just about visible here ( I sprayed them with paint to prevent corrosion ) -
In summary : It's not impossible to do with the roof lining in place, but if your headlining needs replacing it's better to do both things at the same time!
* I just checked the wiring for pre May 1997 cars and it is essentially the same. However, on pre May 97 cars, the interior lights are switched by a 20 Amp(!) relay so you would probably never need to consider fitting a rear relay on those cars - especially if you use LEDs.