Switching the source is a weird and maybe dangerous setup for the mosfet.
If you want to keep the switch there, then at least connect R1 to gate and source, not to gate and ground.
Wise to move the switch to the 12volt supply of the heating element.
The mosfet should be off with 0volt (< 1volt) between gate and source.
Mosfets are ESD sensitive devices. You might have damaged it with handling/soldering.
An old trick is to weave a thin bare wire through the pins, close to the body, and leave it there until the mosfet and the rest of the parts are mounted/soldered.
Leo..