Can you unplug the giant two piece connector from the firewall? If so, disconnect the batt first.
After you unplug it, find the large (12g.) red wire. It goes to the one series 56 spade connector that is larger than all the others in the multiplug.
Check to see if you have continuity between the two brown wires that that terminate at the spade connector with a "U" under it. If you have continuity, then the problem is in the front end harness. If no continuity, then check to see if there is continuity between the large terminal and the one right next to it (also brown wire). If there is continuity, the problem is in the front end harness.
If you do not have continuity in either of those then the problem is under the dash, or in the rear harness.