I do not give up easily but the 300E passed inspection without backup lights and I gave up troubleshooting for now.

Still cannot find the cause of no signal to the harness. The car has been put back together and I'll revisit this issue when it gets warmer here.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor