Where Does My Trailer Brake Wire Go?

[TxQuarterHorse]

I have a 2001 Silverado 1500 extended cab, without any factory towing packages, so I have the 6-wire blunt cut harness under the back bumper.

 

I'm installing a Tekonsha Prodigy brake controller, and I've gotten power hooked up at stud 2 of the UBEC, etc. I'm using the harness w/connector that plugs into the Prodigy and into slot 5 in the MBEC under the dash.

 

The question is, the brake output from the Prodigy (dark blue wire) feeds back into the MBEC at pin 'C', which also has a dark blue wire coming out of the back of the terminal block. Where does this wire lead after it re-enters the wiring loom? Is it tucked up on the frame and terminated somewhere? I need to find out where the end of that is so that I can feed that voltage from the Prodigy to the trailer connector plug.