I resently parted out a 1976 Saab 99 and removed the heated seat inserts
from the driver's side (bottom and back heaters). These have a heat
sensor built into the lower unit. The butt heater has a heat sensor,
built in, which turns it on when your bottom is cold (I don't know at
what temperature). 

No steeenkin control knob required. 

Of course I left it too late to install it this winter like I left it
too late to install my Propex Furnace. I have no heated place to work.
:-(

Find an old Saab 99/900 at a wreckers and get warm.

Larry
'86 Syncro Westy