Also, the 68 Sequentials were NOT Cougar or T-Bird based, rather an aftermarket piece purchased from J.C. Whitney. Able to install just in the trunk, but also made the brake lights sequence, which was illegal in California, New York, and Florida. Cars sold new in those states had the units removed.
The 65-68 T-Bird/Cougar Sequentials were such an interwoven wiring system, it could not be integrated. The 69-70 Shelby's used the Modern solid state trunk sequential unit by adapting the 69-70 Cougar rear wiring harness into the Mustang.