I haven't taken the pickup out, i've just assumed it was connected ok.
Ah, OK! In that case, it's probable that one of the 3 wires coming "from the pickup" is actually a wire connecting the
bridge to ground. And perhaps there's a dry joint on that ground connection.
Then the pickup would have the more usual
2 wires, and may - or may not - have a baseplate. If there's no baseplate, it doesn't matter which wire is hot and which is ground. If there
is a baseplate, then the ground wire grounds both the pickup coil and the plate. It's possible the yellow wire should be the ground wire (although that seems unlikely, yellow is normally used for a hot wire).
You might need to take the bridge off to figure out which wire is which.
Sorry, I'm running out of ideas... :?
(
EDIT: Do you know the exact model of the guitar? It might be possible to find a Fender wiring diagram to see how everything was originally connected.)