Starting 2 weeks or so ago: my 98 xjr:

1. Wiper started randomly turning on/off and parking incorrectly only when activating randomly. Use of the wiper stock turn everything back to normal.
2. Rear light fail stays lit and will randomly turn on and off (not related to rear light activation). Before the issue rear light fail would come on and off about 50% of the time only when breaking i was assuming it was the cheap bulbs i used; all rear lights are good and functional.
3. seat belt warning light will randomly turn on
4. Lights will occasionally turn on and off as wipers wipe when randomly activated
5. all mentioned warning lights turn off and everything works normally, sometimes rear light fail stays.
6. The above usually happens a few min after starting car or randomly cursing, bumpy roads don't seem to affect any of the above

So far i have: tested wiper stock wires while running (jiggled them) saw no and caused no issues. Removed my BCM plug and inspected: everything looked good. Read through FSM to confirm all the above are controlled by the BCM. NO work has been done to my car recently other then me changing my oil. When the issue was just the wiper i was thinking: easy, i'll check the relays and/or replace the wiper motor(park swich), but combined with everything ealse makes me think my BCM is going downhill or i have a really bad ground someplace. Right now i have the battery unplugged for a hard reset. Any input would be greatly appreciated!

 

Most of the time when weird electrical problems occur right after start up it is the signal of low voltage to the modules after the cranking has wore down the battery. The alternator brings charge back up and everything settles down.
I would get the battery load tested. You might also want to clean and polish the battery cable and battery mounts to insure good contact.
Additionally, battery and ground cables can be corroded internally and look good but offer high resistance to voltage and that would affect the modules.
Of of the top of my feeble memory there is a ground strap from the engine to the chassis, a ground strap from the body to the frame (behind right front wheel) and the battery ground in the boot.
Check and clean all of the grounds and test the battery and then let us know what occurs.

 

After start up (few min, majority of the time) or after driving for 10-30 min i have seen the issues. battery is recent: march of this year. Voltage test: Battery off: 12.5 Car on: 13.8, voltage at the engine bay power lug is almost identical to at the battery. I will try and test more locally around the BCM to see if voltage stays similar. it would be fantastic if low voltage to the BCM was the cause!

I am going to try testing for a short between the running lamps and wiper circuit, that's the only thing i can think of that would cause my lights to turn on and off as the wiper goes back and forth. Unless of course my bcm is shorting out internally.

 

Did you see this post regarding the rear brake lights? https://www.jaguarforums.com/forum/x...ar-xj8-151129/

Wiper issue is due to grease with fine metal particles in the electrical contact on the gear face of the wiper transmission. Pull off cover and clean off the dirty grease from the contacts and gear face and it will fix the problem.

See video below on how to fix. It is on a Mercedes, but concept is the same:

 

Sounds like the problem I had. https://www.jaguarforums.com/forum/x...solved-152295/

Running a separate earth from the BPM to the chassis cured it for me.

 

Fixed!

So i had another issue while this was going on: had a water leak on the passenger side of the car. After pulling the floor carpet i laid down baking soda to see the location the leak. To my surprise i had one in the rear (not Sunroof drain) and one from the front right above the BCM. Inspecting closer, a seam in the cowl had failed allowing water to leak into the passenger foot-well bulk head (above the BCM). The BCM connector under the silicone weather strip was full of water shorting the module out when things were just so. Dried the connector out and re-seam sealed the cowl with zero issues or leaks!