Midlife has what you need. As he says the 67/68 Mustang with deluxe Tach cluster has little to no factory wiring/harness documentation. I have a copy of the Mustang Monthly Tech article on installing a deluxe tach cluster in a 67/68 Mustang. My first posting on VMF was the same question you posted here. A VMF guy back then in 2006 was kind to me and sent me this article. If you'd like a copy for added info PM me with your e-mail I'll send it to you.
My '64 1/2 vert. Ordered May '64. D code 4 speed, handling package, caspian blue, accent group, Ford blue manual top.
'68 vert. driver. Owned since Apr '78. C code AT, AC, PS, P disc B, PT lime gold, standard black interior and top. NOS RF fender and left quarter.New top and folding glass.