Tom,
You're getting warm.
It's not a cable retainer but an electrical device retainer or housing mounting bracket.
This was part of the "EZ Order Package" that included this upgrade?
Anyone remember that included upgrade option on the window sticker?
My first Challenger didn't have one on it and I bought it at the dealer's parts department.
No harness modifications were required to make it work, either.
It was a 70's version of plug and play.
Slotts