I have a 2001 Sequoia that came from the factory with flat-4 connector on the OEM trailer hitch. I too needed to add 7-pin receptacle and trailer brake controller. I purchased a 7-way 4-way conversion kit complete with front-to-rear wiring harness from etrailer, p/n ETBC7 (
Brake Controller 7 & 4 Way Installation Kit - 10 Gauge ETBC7 : Trailer hitch bike rack and trailer hitches - etrailer.com). All I had to do in the rear was plug the kit's 4-way male connector to the Sequoia's factory 4-way female connector and tuck that way neatly up under the frame. Then all that was missing was the two wires that were run up front with the included jacketed wire. One wire comes from the brake controller and the other provides ACC power (via key-switched relay) in case your trailer has dome lights, etc. The rest of the install included a Tekonsha Prodigy brake controller (from my Tundra where I upgraded to a Tekonsha P3). I tied the controller's signal input wire to my break pedal switch after reading posts in the towing forum on TS. I also made sure to tie the break controller power and ground directly to the battery as specified by Tekonsha.
Speaking of, the towing forum on TS has a lot of great information for wiring this up. That's where I learned to relay the ACC power circuit to the trailer and also install a 30A circuit breaker off the power lead from the battery.