Right rear tail lamp is confused. Works with lights on but feeds back to another bulb. Past experience says its a ground problem. Ground looks good. Have another S Type (2003), with proper functioning lamps. Will switching the rear module help diagnose the problem? will a used module work without reprograming? can they be reprogramed?
You should be able to swap the modules over if they are of the same age. Certainly, the earlier ones CA 2000 do not require reprogramming.
Bear in mind that the lamps receive a 12 volt supply and to switch them on the module provides the ground. There are five separate ground wires that go to a common ground point. However, the problem may lie in the lamp unit. Worth checking this first before you pull out the trim in the boot to get at the module.
Switched Lamp assembly with same results.