Time consuming but, find high speed wire on switch, remove it, then take each wire one at a time and plug onto that connection. Turn switch to high, if motor runs you have switch problem, if not then you had to go farther up the line. Remove the head cover and check for wiring connections up there. I have found they use a lot of these spade type connectors in trolling motor wiring. If they get slightly loose it causes resistance which creates hat and it will burn connector or wiring right at connector.
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Charlie Weaver USN/ENC 1965-1979