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2014 Sienna LE
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602 Posts
If you don’t want it connected with the van off than you can still power it straight from the battery and add a standard automotive relay inline that’s switched off the ignition to turn the power to the trailer on and off. I don’t have a 4th gen, but I’d doubt there’s somewhere that’s ignition switched that you could just pull a 40 amp load from. Adding a relay is usually pretty easy though.
 

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2014 Sienna LE
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602 Posts
Two more items:

First, I’d recommend getting a vehicle specific trailer light converter, if possible. etrailer had a couple options for my 3rd gen. I haven’t looked for 4th gens for obvious reasons. But if you can get it, I’d do that. The vehicle specific one I got has T-connectors, no need to splice wires, you unplug the taillights, plug into the harness, and plug the harness into the connectors you unplugged. A universal one you’ll have to splice wires somehow, and I’m not a fan of the cheap splicers they usually include. I didn’t see a vehicle specific 7 pin kit, but the vehicle specific 4 pin kit had a 7 pin conversion kit available. This will still require splicing in reverse lights and brake controller. Still much less fiddling with wires though. One option (only option currently for 4th gen Siennas on etrailer): https://www.etrailer.com/Trailer-Wiring/Toyota/Sienna/2022/TK69FR.html?vehicleid=20226003149

And their recommended solution, like my suggestion:
Font Line Material property Parallel Screenshot


Second, while I can’t help you locate an ignition switched wire, I can certainly help you with hooking up a relay, if that’s not something you’re familiar with doing.

Oh, third item I guess. Just checking that you’re aware that in addition to that, you also need to get a brake controller too, right? This will also need power, a connection to the brake lights to know when to actuate, and then be wired to the brake line on the 7 pin connector. I recommend getting an intertial based (proportional) brake controller, not timer based. They will work better. Not as great as a factory integrated brake controller…so much better with my 2020 Silverado and factory brake controller vs the inertial controller on my 2013 Pilot. At least the Pilot had a built-in wiring harness for the brake controller up front, so you didn’t need to run wires from the front to rear.

Hmm, this is interesting. A proportional controller that feeds off OBD data for more reliable, even braking. This could certainly be a lot smoother than one that just has an accelerometer, depending one what data they are able to get out of the OBD data stream. (Factory controllers on trucks and the like use brake pedal position smoothly and accurately apply the trailer brakes for a braking experience that’s like you’re not towing anything at all): https://www.etrailer.com/Brake-Controller/Toyota/Sienna/2021/335DL-100.html?vehicleid=202114766

As much as I sometimes didn’t like it, the setup certainly makes me appreciate the simplicity of hydraulic surge brakes like my boat trailer had, and Uhaul trailers use…
 
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