I have been busy lately and it is hard to find time for projects. But i had a chance to look at this project last weekend.

I did use 2n3055 instead of tip33. But the temperatures still remained too high. I had to use large heatsink to keep the transitor temperature down.
Size of the heatsink.


I looked into e39 front heating setup. It is quite sophisticated setup.
Heating pads run at full 10A current until desired temperature is reached and then the current is pulsating between 0A and 10A to keep the temperature.
Temperature is measured with 10kohm ntc thermistor in the seat base heating element.

I also looked at e38 setup. Instead of limiting current, it limits voltage instead. Same 10kohm thermistor is used to measure temperature and as desired temperature is reached, voltage is reduced to keep the temp.

So I have decided to scrap the idea of creating my own switches and ordered some e53 heating switches.
Like these.


Also i found, that the ebay seat heating pads are very weak. They go hardly over 35 degrees.
There was this guy in German ebay, who was selling genuine e87 heating pads. I can't find the link at the moment, but looks like this is the way to go.