What voltage is your silicone heater? Is it AC or DC? If it's DC why not test it, get a multimeter out and test the resistance between the thermistor wire and the ground wire of your heater. If it's reading around 100Kohm at room temperature and the resistance drops if you warm it up in your hand then you have a bizarre shared ground with the heater. Not sure I'd want to use that. So I'd just buy a thermistor from ebay for £1 and sandwich it between the heater and whatever you're attaching the heater to, in the middle of the bed.
Also check the resistance of the positive wire to the thermistor wire you don't what those to be connected.
While you're at it check the resistance of the heater circuit and stick the resistance and your chosen voltage into a calculator to see how many watts and amps it will draw, does that match up with what you ordered?
