I have a "1968 Cougar Wiring Diagram Manual" that I got on EBay from East Coast Classics a couple of years ago; according to the " 1968 Cougar Instrument Cluster Circuit", the red/yellow-stripe wire (640A and 640B) should connect to the distribution block under the dash near the driver's side door (if I recall, the block is milky white, looks like a Lego block).
The block that you're looking for is 12 posts (2x6), and the red/yellow-stripe wire should attach to the post on one of the corners, adjacent to the red/white-stripe.
I'm trying to describe the orientation without knowing exactly how the block is oriented in the car, sorry....
The other corners should be blue/red-stripe, white/red-stripe, and blue/black-stripe.
I hope this helps, I hope this applies to your specific application (I don't see mention of a tach on this diagram...)