You could make a pigtail and plug it into the trailer connector. When they did my shell last time, they took a 4 wire connector, ran the vehicle side wiring through the access hole in the bed, and then tapped on the trailer connectors for both the power and the brake. Wired the trailer side to the shell. It has worked flawlessly for 4 years. I did go behind them and seal/tape the wires taps they used.
2015 F150 Lariat SCrew, Shortbed, 2.7L EB, 2wd, 3.73 locking rear, Panoramic Moonroof
2014 F350 Lariat SCrew Shortbed, 4x4, Powerstroke
Previous Truck: 2013 F150 Lariat SCrew MaxTow HD Payload, 4x2, EcoBoost