Aftermarket Seat Heat

It's pretty normal for the power lines to come from a solid circuit relay and split to the bottom and back elements. There's only one signal wire from the switch to the relay module so you'd need separate dual sets of relay modules and controls for each side. Either that or interrupt the power after the relay and put rheostats in. With multi level heaters the switch sends a voltage contolled signal to the module. With simple on/off switches it can be a standard bosche type relay.
The switches are just dials with fixed resistors to determine each level so if you wanted to change the levels you could just change the resistors. I've thought it might be nice to shift the entire range down, I don't use the highest setting and often turn it off even the lowest one.

 

Well I finally around to installing my seat heaters. I ordered two kits on AliExpress that had the two controllers on my switch. I installed half of the heaters of both systems. So now instead of left and right the L&R stands for lumbar and rump.

 

From what I understand the double switch for heated seats actually is 10level and does what ur looking for not actually controlling both seats in one switch like most would assume