If the 8 Amp fuse for the overheating switch has blown, check that the switch has moved to its original position before a new fuse is installed.
Disconnect black-red cable from terminal with short circuiting fuse to overheating switch.
Test overheating switch with an ohmmeter connected as shown. The switch must not have continuity.
If, when the heater is cold, the overheating switch completes the circuit, the switch is defective and a new one must be installed.