Well, I had the same problem and since no one ever posts the final solution about what happened, here is what happened to me.
I had the symptoms that the seat heater did not work and I was getting a red flash in the switch when I tried to turn it on.
Initially I switched the switches between the seats.
Next, checked the fuse... I should have done this first. The fuse position is A6.
Then I took out the driver’s seat (the offending seat). It was 4 bolts and a disconnection of a wire bundle. This wire bundle had a clip on it that you had to pull out while at the same time you had to pull the harness apart. Kind of tricky if you have never done it before. The key is to do both things at the same time.
Next, I checked the heater relay. It is the only relay under the seat. They way I checked it way by pulling out the other seat and using the relay that worked for sure off that seat. This is about a $60.00 part on
www.pelicanparts.com. I got the same exact problem telling me that it was not the relay. To disconnect this relay, you have to push the foam up and push in the center of the black fastener (toward you). The nipple will then start to stick out toward the relay and you can pull it all the way out until it stops with some needle nose pliers. After that, you should be able to pull out the fastener.
I then plugged the relay back into the passenger (working) seat and I walked that seat over to the driver’s side and plugged it in. It worked, telling me that it was my driver’s seat for sure.
Then I disconnected the two wiring harnesses that were attached under the seat. There are two wires on each one of these and they both go under the seat foam. I tested the resistance with an Ohm meter of by sticking the red into one of the female side of the harness and the black into the other side of the same harness. I was completing the loop of the heating coil with the Ohm meter and checking the resistance. One had an infinite resistance and one had almost no resistance. This was odd. I checked the working seat and both had almost no resistance. That let me know that I had a bad heating element because there was an "open."
This is where I did some back yard fixin...
I cut the wire after the wiring harness before it went into the foam and I spliced the two wires together. This eliminated the open and the broken heating element. Now the system works, and the seat heats up. I am assuming that there are two heating elements in the seat and the one in the back on my car has broken. The seat still heats the front heating element and the seat appears to heat normally