What year is it ?

My 1972 142 had a relay for the reverse lights on the same mounting bracket as the headlight relay - I think it was the one next to it ?

My 74 model however doesn't use a relay for the reverse lights (a rare backward step for Volvo !)

The actuating switch is on the upper side of the gearbox, LHS towards the front (on an M40, anyway)

I had trouble with mine too for a while - they worked when tested with 12V, but wouldn't light up when I selected reverse. Eventually traced the fault to the switch on the gearbox - the box had obviously been out of the car at some stage, and the wires to it had been re-connected to the wrong terminals .....

There are 3 terminals on the switch. 2 run to the the reverse light circuit, and the 3rd is unused on Australian delivered cars (according to the wiring diagram, the 3rd terminal goes to an ignition cutout switch on US-spec cars .... I assume so they can't be started in gear or something equally stupid)

Hope this helps !

Dave
Melbourne, Australia