A8 and S8 heated seats have two parallel circuits of heating elements (two elements per circuit), one for the back, and one for the bun. If you think your seat isn't heating up correctly, you might want to try this procedure to see if you have an open circuit in one of the elements. The other problem can be a bad thermostat or switch in the dash.
The diagram below shows the driver's side seat heater. You also really need the next page to see where some of the traces go, but I will just explain this one.
First, there are four buses at the top, they are the 30, 15, X and 31. They are all 12 VDC buses, and one is unswitched, one is switched, one is load reduction, and I can't remember what the last one is. But just know that it is either switched or unswitched power.
Then Ground is the line at the bottom. Anything that goes from the top to the bottom is a load and wiring. So you trace from the top to the bottom.
Bottom of Driver's Side Seat
The switch has unswitched power that provides power to the heating element, the control switch is powered from the X bus, which is the load reduction relay I believe. It's switched with the ignition, but not on when the starter switch is turned to "start".
So you could basically check almost everything from either the switch or the green connector under the seat.
You could pull the switch (see audipages iPod installation procedure on how to get to the switch, use two small allen wrenches attached to vice grips to remove the center vent).
Pull the connector off the switch. Using a multimeter set to DC range, measure the voltage at Terminal 1 to ground. Attach a wire to the engine block....it's a good ground and run it around to inside, then put the red lead of the multimeter in on pin 1 and the black lead to the ground wire, it should read 12 VDC. If so, it's good, if not, check ST5, S10, it's most likely blown.
Then check control power. Measure pin 5 voltage to ground with the ignition off. Should be 0. Turn the ignition on, should be 12 VDC. If not, check Fuse ST5, S2 (which is shown on the next page, look at it and follow the trace found on number 14 at the bottom, you'll see it goes to the X bus through the fuse).
Turn ignition off.
Switch the multimeter to resistance. Measure resistance of seat heater elements. Put one lead of the multimeter on pin 2 to ground. If there is an open circuit here, then no current will flow (an open circuit will be shown). If there is resistance, make a note of what it is, probably somewhere 2-3 ohms.
If it has a good reading, then pull the other seat heater switch (which you know works) and measure it's resistance. It should be the same. If it is roughly half of what you got, then one leg is open circuited.
These resistance readings could also be taken at T6K, which is the Green six pin connector under the seat. This is shown on the diagram above.
If you have both control power, seat heater element power, and continuity, then it's most likely the switch or the thermostat in the seat. They control the "electronic on off switch in the actual switch itself".