Some pickups have an additional wire for the center high-mount brake light, running along the driver's side frame rail, near the back. My old '00 GMC Sonoma had one which I didn't know about, so when I installed the cap, I just spliced a wire for the cap's brake light into the existing brake light at the top of the cab. Then I found out about that dedicated wire and it made things a lot easier. Might be worth taking a look.
Geoff & Jill
& Sierra, the little white monster
2013 Ford F-150 XTR SC Ecoboost
2015 Jayco Jay Flight 24FBS