I don't understand why you don't just either snake the wire under the
carpeting or run it on top and at the seam of the step. My 89 Westy is not
the same year as your's so something may be different -- but -- I just
installed an auxillary heater behind the passenger seat and I ran the
wiring under the carpeting (at the step seam) to an auxillary battery under
the driver's seat. It wasn't all that difficult if you know how to use a
snake and you seem to know. Just my 2 cents worth. Good luck.