The manufacturer's blurb should say which wire is the ground wire.

Also the braided shielding around all the pickup wires might need to be grounded depending on the manufacturer.

Here's a link that might be helpful:
http://www.seymourduncan.com/support/how_to_install_a_pu.shtml and

http://www.stewmac.com/cgi-bin/hazel.cgi?action=SERVE&sku=I-4000&PG=2&s1=Free_info_sheets&item=freeinfo/fi.html

AFAIK the tremolo or bridge/tailpiece ground wire goes straight to the common ground whether you use a washer in the cavity or the back of the volume pot.

Don't forget to post us some pics.