A quick update for those who want to use a Ford inertia switch (there are a few). The cheapest switch I could find was from Tasca Ford PN 5L1Z-9341-A Price $37.81 without shipping. You then need the pigtail and Ford gets almost $50 for it. I found it at Rockauto for $27.79 - search for PN WPT1001 (the Ford part number) less shipping.