Interior lamp not working...

[dorsetdave]

Brilliant set of explanatory pictures. Thanks very much.

[Robsey]

If you look at the second picture, you can see two grey wires straggling along the sill and across the front of the driver's seat.

If it helps the carpet is in two pieces.

Front half and back half...
and the join is on the box section that runs across the car underneath the front of the front seats.
Just where the wiring turns inboard from the sills.

[dorsetdave]

Snowy, my problem resolved. Exactly as per your solution. The connector in front of the driver's seat had corroded. I soldered in new wires and everything works as it should including the Factory alarm which has never worked since I have owned the car.
Thanks to all with their advice.