Question How to convert 4-way trailer wiring to 5-way on 2004 Sienna??

[TC_2003_Tundra]

I would like to add trailer brakes to my boat trailer. In the past, I have owned a boat trailer with disc brakes and a electric reverse backflow solenoid, that prevented the brakes from engaging when the vehicle was in reverse. This set-up worked great on the Tundra with the 7 way wiring.

I would like to convert the trailer wiring on my 2004 Sienna from the 4-way to the 5-way with the backup wiring. Since the backup lights are located in the back door and not part of the turn signals (that are easy to access), I am scratching my head trying to figure out the best place to slice into the reverse light wiring.

Has anyone done this already? I did try to search but only found 3-4 listings with anything about backup lights.

Thanks in advance your help.

Todd