how to fix reverse light switch on Citroen Picasso?

Replace it with a new one. You haven't told us which engine but it probably just screws into the top of the gearbox. Undo the wires to it (there'll be something like a spring clip), unscrew the switch using a suitably sized spanner. Screw in the new switch (they cost about £8 from your dealer), reconnect the wiring.

ChicksFan
June 2013