The Propex line of heaters is designed with an onboard fuse to protect the heaters’ sensitive central circuitry. Consider this fuse your heater's front line of defense against an over-current condition for your heater. 

To protect this fuse, it is imperative that there is no 12-volt power to the unit when connecting the thermostat to or disconnecting it from your heater unit. In other words, connect the thermostat and THEN connect the 12-volt power. If this sequence is not followed, the power surge on re-connection may cause the fuse to blow. 

A sure sign your fuse has blown is that there is no initial fan noise when connecting the unit to 12-volt power. In normal operation there is a half second whir of fan noise when the unit first gets power. If you do not hear the fan, consider the probability that your fuse may be blown.