HI BartTemporarily attach a jumper wire between a good ground and the taillight housing. I've found the '65-66 taillight housings to be notoriously poorly grounded.
Hint: Years ago I made myself a jumper wire that I've kept in my toolbox for probably 30 years, now. It's an old telephone handset cable with all the conductors wired together and soldered and terminated on each end with alligator clips. Easily expandable to 3 feet. Always a bunch of 'em to be found floating around.
yes I have 10 feet of 16 Ga . I will have to do this the hard way starting from the battery and following the schematic.