I had the same problem a few weeks back (passenger side brake light not working, only 7volts) and posted on here in tech talk. The brake wiring travels from the switch at the brake pedal, down the transmission tunnel and to the drivers side rear wheel arch where it branches off at a connector all wrapped up in tape (well, mine was) and heads across to the passenger side. In my case I unwrapped the tape on the drivers side, replaced the connector and cleaned the area up and then had both working brake lights again.
So, have a peek in the drivers side wheel arch and look for the wiring coming through to the back light.