Hi everybody...... got something really weird going on with my '88 Jaguar XJS dimmer switch.
Electrical of course.
I put a new ignition switch in which was a frustrating ordeal to say the least.

The instrument cluster goes all whacky, and says the temperature is HIGH, fuel is HIGH, battery charge is LOW.....and the button for the windows won't roll the windows down. And when I press the window button all the guages start flying around!
Then when I can get the lights on low beam, all the guages return to normal!
Like what the hell us this all about!?
I got a new dimmer switch relay and gonna put it in.
That's the large one on the driver's side near the fuse box.
I bought it three years ago from Welch Enterprise but lost it during several moves, and just found it today!
My mechanic says the aftermarket ones aren't manufactured anymore.
I saw several posts online from Jaguar owners who are having similar issues.
Your best bet is a salvage yard.
You can use the dimmer switch from an 87 XJ6 for the XJS V12. They are the Hella switches, they are the same.

 

jetlag700, I had a car that was doing something similar to what your car is doing (it was a dodge, but of a similar year). What I found was the ground wire for the cab had started to deteriorate and it was causing a high resistance. It was fine when only the engine was running or I ran something small inside the cabinet. BUt, if I put multiple things on, the car freaked out. I would tell you to run a temporary wire between say the firewall and the engine. See if that makes things better. If it does, then your ground wire is acting up. IT could possibly be the power wire into the cab too. The only way I was able to find the problem was I stressed the system and turned on everything. I suddenly got a new light on in the engine bay. WHen I looked, it was the ground wire glowing white hot, right next to the fuel line. Needless to say, I had a serious case of puckering when I saw that.

As for the switch, if the switch has the same sort of outside and you know the internals are the same, then go ahead and use the switch.