Still in week one with my XJR and trying to resolve the issues it arrived with, and electronics is next on the list. Here are the symptoms:

• When the car is off, keys out of the ignition, locked or unlocked, the lighting behind the stereo keys remains on
• The SRS light remains on all the time when the vehicle is running
• The seatbelt warning does not work
• I haven't experienced it yet, but the previous owner said it would not hold a charge more than a few days due to the security/locking module (we'll see if it behaves, as it has so far)

It would be rather too optimistic of me to think these all share a root cause, but I figured best to put them all here rather than in separate questions. Any ideas/recommendations are welcome, including to take it to a pro - and if that is your recommendation, names/contacts in the Atlanta, GA area are much appreciated.

 

Additional electrical glitch: automatic headlights don’t work.

 

I would recommend a battery tender to keep the charge up on the battery, especially if it is sitting for periods of 10+ days. If you don't, you can nuke a cell when voltage gets too low, if not already.
Have you done a hard reset?
It may be apparent your SRS and your seat belt connections are of the same source and issue.
I am suspect of someone attempting to do one thing and messing up something else in the process
Review the grounds in the PDF as well as the hard reset and we'll go from there.
Your headlight issue may be the rear view mirror sensor?

 

No hard reset yet, but that is definitely in the cards to do soon.
I figured the SRS/seatbelt issue was related, but separate from the other issues. I am almost positive the headlight issue is from the rear view mirror, as mine is shot.
Thanks for the PDF - I’ll give it a look!

 

Hard reset done, and the SRS light (previously intermittent) is now out, but still no luck with the headlights, seatbelt warning, or mysterious persistent backlights. Also checked the battery: 12V off, 13.6V running, so not likely an issue there. Looks like some sensors/connections/modules need some checking…

at least it drives well!

 

The battery usually maintains for a few days, then can drop, depending on age and condition. It can't go below 11.4v or the ghosts start to take over.
For the SRS, lets see if that lasts or not.
The stereo lights could be a bad rheostat. A local radio repair shop should be able to deduce that easily enough and not to costly either. Have you had the unit out to see whether someone has hacked into a wire with another one?

I've attached the 1999 electrical guide

 

Haven’t popped the radio out, as I’ve been planning my eventual approach to get phone connectivity and would do it then. Maybe I’ll pull that forward.

 

Update: turns out the radio had an internal fault, causing the lights to stay on and drain the battery - all fixed with a replacement unit. The only lingering electronic oddity is the lack of seatbelt warning when the car starts, which I have a feeling is actually a fault in the latches (from what I've seen written in other threads). Thoughts on that are welcome, but once that's sorted, it's only the cosmetics left!

 

Seatbelt warnings should not be too hard to figure out. I is good to get
things to a point you can manage without dread.