Thermostat Switch - Circuit On At 100°F and Off At 85°F - Large Flange

can I use this on a Motor home 12 volt vent fan on the back of the refrigerator?

I'm using this in 12v system. Should the switch be wired to the positive wire or to the ground wire?

When this Switch Fails.... Does it stay open (off) or closed (on)?

I want to use this to control a fan where the plug is a transformer from 120v to 5v. Should I install the switch between fan and plug or plug and wall