I am adding Speed Control to a 67 Mustang that has every other option on it. Apparently, for 67 Mustangs (and Cougars?) with the Convenience Control option, there is a ‘proof’ wire that runs from the relay to the wiring harness under the dash. It powers the Low Fuel light for a few seconds to ‘prove’ that the bulb is working. It has a ring connector at the relay and a three pin female connector under the dash. It runs about 110 inches long. The basic part number is 9C877.
Now, the odd thing is that the wire is black on a Mustang and red on a Cougar.
The Parts catalog calls for the same wire when using Speed Control, so it follows that if you already have convenience Control, then you just plug your Speed Control into the 3 pin female end.
Since my car came with Convenience control, I should have had that wire… But the previous owner cut it and ran the power to the ignition. That really sucks. At least, I have the end pieces and I can splice on a black wire to make it work.
If anyone has a good black wire, let me know!