put it on 20v, make sure its working by using the + and - on the battery for 12v ish or 14v ish if its running.

as long as it hasnt got Xenons take the good side`s wiring plug off and check for + and - at each terminal, draw a little diagram and what voltages there is and if its + or -.

then test the non working side.

same goes for rear lights also.

then its wiring diagram time, doing a continuity check (i.e wire is not broken!) etc