I found this car seat heating
pad at a thrift store a couple years back thinking it might come in
handy for something farm-related in the future.
There's an interlock switch
that tells the heater if anyone is sitting on it or not which needs to
be bypassed for this application. Hook up a 12 volt DC power supply and
you've got yourself a homemade do it yourself heating pad for
sprouting sweet potatoes and anything else that needs to be kept warm
during these cold spring nights.