While waiting for a different RS3200 module to arrive (Toyota made a million variations of them), I put some work into installing the rear heater. In doing this, I found 1000 MUD posts on DELETING the rear heater and exactly 1 post on ADDING a rear heater. So, I guess this will bring that total up to 2 now.
My truck didn't come with rear heat, so I cut the hole in the floor. I extrapolated measurements based on photos from MUD. Got pretty dang close. Off maybe 3mm.
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I found a rear heater off Yahoo Auctions super cheap, but I quickly realized it was a 12V edition. So, I found a 24V motor on PartSouq (87104-90A04) and a 24V resistor from CityRacer (989337-6620). The resistor was the right part, but it was too many ohms. The fan would not start on its own in HIGH. So I installed a 12V resistor from NAPA (ECH BR14), and all is well. Strange.
There was a spot in the steering column relay bank just waiting for the 4 prong rear heater relay (Niles relay 4CT-1M)
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I moved the rear heater switch up to the front (Connector Experts CE4018 and CE4337).
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Then, I installed a USB charger in the back (Blue Sea Systems 1039). I chose to tap into the rear lumbar power circuit. It's fused and has plenty of juice.
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Next up is to run the lines from the rear heater core to the engine block. If someone could snap some photos of where the lines enter/exit the engine block, I would be grateful. The drawings are easy to find, but they are a bit unclear.