No reversing lights - 1996 850 estate

Assuming that the bulbs haven't failed, you need a wiring diagram (Haynes Manual, etc.) to be able to trace all the components in the circuit.

They're almost certainly fed off an ignition controlled relay, feeding through a switch on the gearbox or gear linkage which detects when reverse is selected. Any of these could be faulty, or a wire could have fallen off somewhere.

If its a manual- the switch fails often and is located on top of the gear box, directly below the air filter housing. You'll need a deep 28mm socket to remove it.

Tim..