I have a 91 XJS convertible. The dash gauges all work fine. The backlighting does not. Nothing illuminates when the headlights are on . The dashboard rheostat does nothing. I'm supposing it could be the rheostat or bulbs of some kind. I don't want to disassemble the dash until I have some idea what to look for and where to look. My manuals offer little or no assistance. Can anyone help.

Thanks,
Jeff Ford