It is fairly common for someone to improperly route the cable going to the back of the headlight bulb.

This happens when the bulb is changed or if the car was in a collision and the front was worked on. The headlight cable/wires should come up between the outside of the casting and the fender, then over the casting and have a 90 degree bend and plug into the back of the bulb. When routed this way, the cable twist along a 6-8 inch length.

What often happens, the cable is routed directly up behind the bulb and each time the lights open and close, the cable is flexed at the connector, over time, this breaks the strands of wires inside the cable and eventually you have no connecting wires. If this happens on the passenger side, you loose only the passenger side light. However if it happens on the drivers side, both lights fail.

Photo attached which shows proper wire routing.

Thanks to Barney Eaton, member of the Reatta forums, for this information.