Here is an Ebay auction for an NOS C5ZZ 15520 A backup light switch by Ebay member "aboss429ataol" with a Buy-It-Now of $199.99:
This auction is for a NEW ORIGINAL FORD 1965 1966 Ford Mustang or Shelby GT350 4 SPEED Back Up Light Switch. It is a New Old Stock piece with Ford part number C5ZZ-15520A .. This part is obsolete and hard to find.
https://www.ebay.com/itm/NOS-1965-1…5.c100012.m1985
-Fred-