Probably the switch on that door.  The switch has 3 wires. One is the
supply feed (from battery) and is usually the center wire.  The two
outer wires are for LOCK and UNLOCK.  The switch is probably bad on
one side but still good on the other.  

Replace the switch, but before you do, be sure the wires are attached
and not corroded where they connect to the switch.  If you want to
test the switch, take a volt meter or test light.  The center terminal
should be hot all the time.  The outer 2 wires should be hot when the
switch is flipped one way, the other way, the other wire is hot (not
necessarily the same wire toward the direction you flip the switch.

The only other possibility would be a broken wire somewhere, but I bet
its the switch.  

Mark