Thanks, I have the same plastic cover as well. I thought maybe this piece would go where the wires pass behind the battery but I can't see how it would attach or stay there.Have one similar on my 74 to cover the wiring just under the washer bottle. My 73 was kinda hidden by the washer bottle but you can see where the wiring sneaks under in the cover.
Yes, very much like the wire channel under the carpet.Looks like the piece under the carpet, on the inner rocker.
The mystery continues!never have seen that on my 74 Charger