They are turned on (low) by the pressure switch on the filter/dryer, through the ceramic resistor next to the filter/dryer.

Turned on (high) by the temp sensor/switch on the head.

Inadequate freon fill will not allow them to turn on, the most common problem is a cracked resistor.

Is the "low" resistor held in place with a metal clip? I alo see 4 wires (2 on each side) held together with 2 white plastic clips and something else that's round at the base with 2 connections going to it.