Here'sthe way I'd do it. First Go to NAPA and pick up 5 Weatherpak connectors(late model GM type.) You'll need the 4 or 5 socket ones. Inspect your harness for how many wires you have. Discount the brown wire in the harness cuz thats the fuel sender. At the left rear corner is where to locate the harness.
What you'll do next is cut the wires on forward side you'll install the pin side & the socket side on the rear harness. This will give you a clean disconnect point.
With the other 3 connectors you'll make a splitter. One end to plug into original harness forward side,one to connect to rear side of harness,and the last heading to the trailer disconnect.
Confused?? I'll email you a scan tomorrow.
Yes a heavier flasher will be needed