My tail lights in my '94 XJ6 have been giving me issues. Sometimes they work, sometimes they don't. I tested and confirmed that the power is getting to the relay module but it's like the module isn't being sent the signal to flip them on. I kind of thought maybe it's the headlight switch on the steering column that is bad but it turns the headlights on with no problem and from the wiring diagram in my Haynes manual it looks like the front and rear lights are wired to the switch on the same red wire so it would seem that if it was the switch then the headlights wouldn't work either. The rear fog lights work, brake lights, turn signals, reverse lights but not the tail lights, license plate lights or any of the side lights. Anyone else have any war stories to relate that might shed some light on this?

 

If you remove chip from module and clean connections might help. Should be a module on each side.

 

OK. Tried that. Didn't appear to change anything. I didn't think it could be that anyway because like you said there is a relay module for the left lights and one for the right lights and it's unlikely that they would both fail simultaneously. The problem has to be happening somewhere before it gets to the relay modules.

 

See this post for what worked on my Sovereign today:

1990 Jaguar Sovereign : No Tail Lights or Parking Lights | AndyRupert.com

Perhaps this will help you as well.

Andy