You can also get flat-two connectors which would be perfect for this as you're only supplying power and ground to the heaters since the switch will be onboard the seat as well. The flat-two's should be easier to hide under the floor or trim panels too.
Excellent suggestion - I hadn't thought about using those but something has come up that might mean that four or even five wires to each seat will be required - two wires to the seat heater and two or three wires to a remotely mounted seat heater switch.
I've spent countless hours reading forum threads on installing seat heaters in front seats and watching "how-to" videos on YouTube but I haven't found anything on doing it in 2nd row seats. A challenge in many vehicles including the Sienna are the upholstery "listings" - the upholstery seams that attach to metal rods with hog rings to hold the upholstery tightly in place.
Some who have installed seat heaters in seats with "front to back" listings like in the Sienna have not reattached the center listing in order to avoid cutting a slot for the listing in the carbon fiber seat heater panel. That method may be "good enough" for some but it's not for me - looks too sloppy. The Dorman seat heaters I had planned to use are square, aren't that large and might require cutting slots for all three listings. I now think that a "longer and narrower" carbon fiber seat heater panel would work better since it would just fit between the two outer listings and require cutting one slot for the center listing. A long/narrow carbon fiber panel would also leave enough of the electrical circuit intact at the front and back of the seat cushion and the bottom and top of seat back for adequate electrical conductivity after the one slot is cut.
I took my Sienna to a professional installer a week ago today. Oh ... my! He's even older than I am and we mainly talked about why we still wanted to work past the age where most have retired.
The installer had experience installing seat heaters in the second row seats of SUV's but had never installed them in ottoman seats like the ones in the second row of the Sienna Limited that slide forward and backward so much. He pointed out that the plastic trim panels on the sides of the ottoman seats are different than the ones on the front seats. The front seat cushion plastic trim panels are essentially formed slabs of one layer plastic at the front corners while the front corners of the plastic seat cushion trim panels on the 2nd row ottoman seats have two layers with a void between the two layers. There is plenty of clearance for seat heater switches in the corners of the front seat cushion trim panels but maybe not in the 2nd row ottoman seats.
I wanted to have the round seat heater switches on the inboard front corners of the ottoman seats but the void between the two plastic layers may not be deep enough for the switch and the connector from the kit's wiring harness. There is significantly more space between the two plastic layers on the outboard corners of the seats but mounting the heater switches there might interfere with the leg rest release lever. The main reason I wanted the seat heater switches on the inboard corners of the ottoman seats is so I could see from the driver seat if someone had inadvertently left them on and be able to reach back and turn them off.
The installer told me that he would rather put the seat heater switches at the back of the center console ... until I showed him that the rear of the center console slides forward and backward. He was concerned that he might not be able to hide the wires for the seat heater switches in the sliding console until I mentioned that there must already be wires to the back of the sliding console for the audio/video inputs and the 110V outlet. He also said that he has carbon fiber seat heater pads that come pre-cut for the center listing. Someone else at the business told me that they use seat heaters from "Roadwire".
So ... the installer said that he would do the job for $700 if he could put the seat heater switches at the back of the center console. I had originally wanted to do the wiring myself but I can see why he wants to do the entire job to ensure that everything functions properly.
I really, really don't want the 2nd row seat heater switches in the back of the center console including for the reason that they would be impossible for 2nd row passengers to reach when the console is in the forward position and the ottoman seats are in their rear most position. My next step is to use my borescope to see how much clearance there really is in the voids in the ottoman seat cushion trim panels. I need to know the exact dimensions of the switches and their connectors which I assume I can get by asking on Amazon.com. If I could wave my magic wand, I'd put the seat heater switches just aft of the power window switches in the sliding door but that's not realistic.
The outside thermometer in the Sienna read zero degrees Fahrenheit this morning on the way to work. It was warmer in my garage but still too cold to mess with this project.