2004 Jaguar XJ8 60,000 miles

When the headlights are ON or on AUTO, the rear lights (tail, reverse, turn, brake) do not work properly. Improper lights flash, some dim, some go brighter. If the headlights are turned OFF, all rear lights work properly. We believe the problems trace back to water in the trunk, which shorted out two plugs on the fuse box leading to an alternator failure (it began putting out 18 volts). The alternator was replaced as well as the fuse box at which time the rear light problem was noticed. Also, when the car is put in reverse, the A/C fan stops. The rear and front body control modules have both been replaced with no change. The car runs perfectly otherwise.

Any Ideas?

 

sounds like a ground problem to me. probably the grounds behind the headlights. Please update after you find the reason.

 

Yes, it should be a ground issue.
The Rear Electronic Module switches the ground to the light units. REM is located in the right rear trunk and has 3 ground cables. I would check these.

 

+1 on checking your grounds

 

Thanks - we'll recheck the grounds

I should have added to the original "When the headlights are ON or on AUTO, the rear lights (tail, reverse, turn, brake) do not work properly," that the rear light problem occurs with the AUTO switched on even when the headlights aren't actually on (during the day).

 

There is a light detector to look for sunlight in the climate control system on the X300 on the dash

Would have to look if on your model if the detector and it's wiring has any inputs to the climate control and the exterior light control

Yes it does , referred to as the autolamp sensor on page 93 upper left corner of the page , it will have a White / Black wire and another Brown / White colored wire powered by fuse F14 in the passenger junction fuse box , this must have the switched system power relay # 1 to close to power the fuse

The autolamp sensor has a ground point to look at with the black wire

You have a separate wired climate control dual solar sensor on page 81 top right of the page , it will have 2 Orange wires and 1 Blue