My Audi A4 (1997 Quattro S-Line, 2L Diesel) has developed a few electrical faults over the last couple of days and I'd like to ask whether anyone would have any ideas as to the cause?

1) Reverse lights are permanently on as soon as key is turned in ignition. Other lights (headlights/indicators etc) are working ok and the fuse was replaced yesterday.
2) Heater/air conditioning panel is permanently off and won't switch on (no lights or power).
3) Electric windows still work when the key is completely out of the ignition.

There are no warning lights on and it's driving ok but I'm just a bit worried that things may deteriorate further. Apologies for the lack of technical terms but I know very little about car engines or components. Obviously, it would be best to take it to a garage to get looked at properly but I can't afford that right now so I'm wondering whether anyone has experienced anything similar and had it fixed or if anyone had any ideas what may be causing these problems?