Not sure on a 308 but on an 807 the sidelights are fed from fuse(s) and a relay in the BSI. Pull one of relevant fuses and check using a probe that one of the contacts is receiving 12v when switched on. That will prove the switch, comms and BSI are good. Then go onto Peugeot Servicebox (register and pay a few Euros) to down load the BSI wiring diagram which will show which plug and pin numbers the side lamp feeds exit the BSI. You can then probe these wires to prove they are live when switched on and follow the wiring down to the lamps to find the break. If it all the lamps will be close to the BSI, check its plugs are not loose or wet.