Use a sharpened coat-hanger wire to punch (carefully!) through the rubber wiring dam behind the battery, then route behind dash, and under the carpet alongside the tranny hump, and exit between the carpet sections at the back seat footwell, or through one of the rear deck carpet openings used by the rear seat hinge mech.

The rubber dam will seal the wire pretty good.

Keep the wire inside.

Oh yeah: put an inline fuse at the battery end, don't run an unfused length of wire into the cab.

Don `87 Mitsu 2dr, Rubicon survivor, GModified.