If the nichrome wire has broken at the t-stat tab, solder a 2"-3" pigtail to the wire first. This will relieve the stresses caused by a too-tight wire when a too-fat fanny pushes down too-hard on the grid.

As I recall from working on my bun warmers, the wire for the grids sneaks down under the carpet from the center console (where the switches mount) and out through the hole in the carpet at the floor vents. I'd guess that the harness is an add-on, and simply connects from the switches to the fuse panel. (You may need to "harvest" that, too, from the organ donors.)
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Don Foster (near Cape Cod, MA)