Interior lights not working
If you have an 80s XJS with the fuse blowing out repeatedly or no lights after replacing the fuse, you may have a bad rheostat switch or something similar to the case I had soon after purchasing my 87 XJSC around 12 years ago.
I bought this near-show car that was always climate control garaged and upon close inspection, I discovered that the passenger door lamp switch was loose. These are the buttons in the door jambs that pop out when you open the doors. There are fastening tabs that are soft and thin and over time can get loose.
Because I did not fix it right away as my business was rapidly growing, I parked the car in my warehouse with my other cars and eventually forgot about it. Then months later, I was driving the 10 hour trip from my South Carolina home to my Greenwich, CT home and stopped for dinner in Roanoke VA. refueled as it was getting dark, turned on the lights and all interior lights were out. I bought a box of fuses and blew them all. Well anyone who owns an XJS not to mention a V12 knows that we have a hard time behaving ourselves on a highway at night with very few cars and even fewer cars with a uniformed person driving. I had to have something to show my gauges so as truck stops have everything from a kitchen sink to hiking boots, I found a set of lights that fastened to any surface by two-sided tape and powered by the lighter. I used them for the next two weeks but the problem was still there and would blow the fuse as soon as I would open a door. Someone had installed a remote lock to the car which I wanted to remove and then thought it was causing the problem. So after pulling both fuse boxes out of the car to closely inspect, I put it all back together by the end of this week-long fiasco and had a stiff drink or something a little stronger than water while scratching my head when I finally had a total recall of the loose door switch.
If it is the rheostat switch, here are some procedures to troubleshoot this.
The current for the dash lights goes through fuse #7, 10A, in the main fuse panel above the driver's footwell, then through the rheostat or dimmer switch which has a large plastic rotary knob on the right side of the driver footwell knee bolster or under-dash trim pad, and then to the dash lights.
Try turning that rheostat knob through its full range of travel from side to side several times to see if that cures the problem.
If not, remove the knee bolster to which the rheostat is mounted and do the following:
(1) check for voltage (with the light switch turned on) at the Red/Blue wires going into the rheostat;
(2) check for voltage in the Red/White wires coming from the rheostat;
(3) If you have voltage at the Red/Blue wires, but NOT at the Red/White wires, the rheostat is bad, or at least needs cleaning.
You can connect the Red/Blue wires to the Red/White wires to wire around the rheostat (full brightness) if it can't be fixed.