The flasher itself is held to the right of the glove box on the inner edge of the lower dash by C1TF-13351-A, bracket, 45144-S2, spring nut, and 42127-S2, screw, per the 1966 Mustang Electrical Assembly Manual, page 24. The bracket is a "universal' type in that the flasher can be inserted horizontally or vertically.
I suggest you get a set of 1966 Mustang Assembly manuals, plus the 64-65 Chassis. Those will answer 95 percent of your questions.
Jim