Hey Homebrewer and Noseoil!
Using the 3rd brake light converter as an input to the trailer-mounted brake controller, do you have a problem with the brakes pulsing if the TV hazards/flashers are on? I was thinking about doing something similar, possibly using relays, when/if I mount the brake controller in my tongue box.
As of today, I'm leaning towards tapping off of the brake pedal switch to feed the brake controller (mounted in the tongue box) via the blue wire on the 7-pin. I believe doing so would still allow any TV with a 7-pin and dash-mounted brake controller to pull this trailer and have operational brakes. Note; I haven't considered this enough yet to be comfortable saying it
will work. On the flip-side, doing this wouldn't allow a TV with a 4-pin to pull this trailer and control the brakes, but the trailer won't be heavy enough to require it and a 4-pin isn't meant to control brakes anyhow. Also, our TV, which is a Dodge Grand Caravan wouldn't be able to pull a trailer using the 7-pin connector, but it also has a 4-pin and it will likely never pull another trailer with brakes or a 7-ping connector for that matter.
With that said, one of the reasons I'm leaning this way is because I plan on installing turn-assist lights on the front of the fenders - essentially forward-facing white rock lights that turn on when the blinkers are used. I can't do that using only the yellow and green wires without some funky things happening if the brakes, blinkers and/or hazards are used simultaneously... so I need the brake pedal switch signal to solve this 1st-world problem. The forward-facing turn lights and rear-facing reverse lights (also mounted on the fenders) will act as "porch" lighting when we're camping.
I still need to draw some schematics and wiring diagrams, and do a little more reading on brake controllers before I decide which way to go.
Steve