The bulb failure unit is in the boot, on the passenger side, behind the boot light panel.
There's two units behind there. One is the fuel pump ECU, the other is the bulb failure unit.
IIRC, the fuel pump ECU is metal, the other is plastic. (Could be wrong though). The fuel pump ECU has a heavy blue + blue/red cable going to it. Obviously, this is the one you don't want.
Bear in mind I'm only presuming this is where the problem lay, as all the ones I've looked at, have power for the rear lights travelling to the back.
Don't blame me if you rip the boot apart just to find out that isn't the problem