My LX 450 came with “factory” heated seats, but it was clearly a dealer or third party install and definitely not up to Toyota standards. There was a thread/ post here recently about it and someone posted the Toyota factory schematic for an overseas variant. It’s a big wiring job to do it correctly.
I have messing with the dash lately on wiring up my ham, and this would be my advice on wiring it up.
1. Find a Toyota sealed relay unit from an auto recycler. Over the years, I found several that just mount on the battery positive post. Had to cut a hole in my battery box to feed the relay battery post mount through. Mounted another style relay to a bracket in the engine compartment. That is what I did for my driving lights, etc. A relay with larger wires is required - 20A min. The schematic I spoke of earlier (attached below) shows both seats (each seat has a lower and upper pad), being feed via one 20A fuse.
2. As others have said, keep the fuse (use a LittleFuse in-line sealed design) as close to the battery as possible. Could be before or after relay depending how close the relay is to the battery. Use quality SXL Automotive type wire and not cheap dollar store wire. I highly recommend using Deutsch connectors even inside the truck as they are compact sealed high quality and easy to unplug. I redid all my “factory” seat wiring using these connectors. Use wire protector on the portions in the engine compartment. If you on adding more electrical features in the future, add more wires in the same bundle for further use. Mark the ends, and draw up a schematic for future reference. I just had to make another run to my aux battery for my HAM radio. Would have been nice if I already had extra wires already run, or even a terminal block inside the truck I could use as I need when installing more electrical items.
3. To do this correctly, I would pull the centre console and shifter console and run all wires from/ through DS firewall, under the dash/ steering column to the centre shifter console then to centre console, and then under the rug up to under each seat. To pull the centre dash fascia, I first unscrew the two top screws where my cup holder is or whatever was there originally. Then there are two screws where the ashtray is I believe (remove mine to install my Cobra CB unit there). The facia will then pop out. Start from the right side and move along the top as there is a clip in the top left corner. There is another centre clip, and one n the lower left corner. To mak it easier to get at all the connectors on the backside, I block one of my tires, and place the shifter into low. Best also to throw down a towel or something soft to protect the fascia. I would also recommend taping up the dash along the left side next to the fascia platevto help avoid scratching it up while you play with removing the connectors. To remove the shifter console there is one Philips screw and one 10 mm bolt on each side of the console. Remove the t-case shifter knob. Pull up and press the trans shifter button to get it past. The centre console uses 3 x 10 mm bolts. Two are at the rear (one each side), and one at the front on the driver’s side. There are also two M10 nuts under the mat in the bottom on the console. IIRC these were special torx or other type nut, but I replaced mine with regular nuts. The LX has the centre CD changer so there is a small fascia surround plate. You need to remove if before lifting the centre console. Gently pry it off from the top with a small flat headed screwdriver or trim removal tool.
This should get you started. If you need the Factory heater switches, I have at least half a dozen various spare sets I could sell you at a reasonable mud price. Just PM me.
View attachment 3117047
. It looks like the LX450 doesn’t come with the proper wiring harness. 
This job seems to be more intricate than what I was expecting.
