Sounds like you have the pickups installed but need to wire the ground of the bridge (the bridge of the strings on the body, not the bridge pickup), correct?
Assuming all the rest of the wiring harness was done correctly, the string bridge ground could be soldered to the back of the volume pot if it reaches or one of the other tone pots if they are closer, or to the output jack directly. I would not attempt to wire the string bridge ground to the bridge pickup wires.
Theoretically, if the string bridge ground wire was removed completely or was cut so short it won't reach the wiring harness, you could solder a jumper wire to the back plate or cover of the bridge (if it has a cover), which should be grounded. But that's not a great solution IMO. I wouldn't want to heat up the back of the pickup just to solder a wire on there.
If the string bridge ground wire was removed completely, I believe you're going to have to take the bridge off to install a new one. If the bridge is a ABR type bridge, usually the string bridge ground wire is wedged between the studs and the body wood, if I am not mistaken. (Someone can correct me on that. I've never been in the situation where I had to replace the string bridge ground wire.)
I've never heard of grounding to a strap lock - that would never work unless you ran a ground wire from the strap lock to the output jack..