I bought a used 2015 certified F150 with extended warranty. It came with the towing package, but no brake controller. If I install the ford brake controller myself and have the dealer turn on whatever needs to be turned on. Would I void the warranty because I installed the controller myself.

 

No.

And you can turn it on with FORScan.

 

No, and there's nothing hard about installing the integrated trailer brake controller (ITBC) yourself. Since you have the towing package, there's a plug under the dash near the steering wheel. The ITBC is Plug&Play, But it won't work until you or someone tells the PCM (computer) that you now have an ITBC. "Turning on" the ITBC is a 5-minute job for a good Ford tech, or you can DIY using FORScan. FORScan for Windows is free. https://forscan.org/download.html

 

Thanks, I will give it a try.