Solution for SR without 4 pin towing connector at rear bumper

Any interest, comments...
The solution does NOT address back-up, battery charger or electrical trailer brake circuits.

 

I don't understand what you're asking. The 4-pin plug has never been about anything but brake/turn and running lights, goes way, way, way back. The 7 pin came about to address evolving needs of bigger trailers.

 

"History" lesson not withstanding, for those who have a plain jane SR, they lack complete towing harness features.
This harness would connect to the 4 pin connector in the cab (hidden) then routed under the sill then under the driver's seat. Through the grommet (to be sealed) then along the vehicle harness to the rear bumper.
Owner would remove the blank cover and install a 7/4 trailer assembly, purchased (not provided).
The added harness connects to the newly installed OEM towing assembly.
The OEM towing assembly (internally) shares turn & stop, tails and ground from the 7 pin to the 4 pin.
So, as before mentioned the 7 pin if used would lack reverse, battery charger and towing brake circuits, only the features of the 4 pin are available.
Adding the missing circuits wouldn't be rocket science though.

 

Reverse harness to provide said circuit to T12. Fuse holder as OEM to prevent loosing reverse circuit of vehicle if trailer has a mishap.