The guys above have good advice, but if you are intent on using the stock wiring, you would tie into the tan/white wire on the left side of the bulk head connector in the engine bay. That wire goes through the bulkhead, through a fuse, and back out the bulkhead with the rear wiring harness, and splits to a connector that runs to the dual tank switch through the firewall. The other side of the split goes to a plug with no terminated connector in the other end, on a dual tank setup (if it was a single tank, this would be terminated for use on the driver side tank). For what it's worth, I know several people running the stock wiring with no issues. That's not to say the upgrade isn't worthwhile for peace of mind!
You can see the wire a little bit here (tan/white spliced to gray). Pardon the mess, I haven't cleaned everything up.
