It is the K-10 heater module. Seems odd it would work sometimes and not others. If you see a kw draw on the power screen when requesting heat, then the module is drawing current and working. It's a bitch to change and you have to disable the HV system as it's fed by 300v from the battery pack. You also have to remove the engine intake manifold to get at it, mounted on the firewall. Actually your problem sounds more to me like the switching valve which has also had some problems. Or possibly the coolant pump. Replacing any of those 3 things also requires draining and refilling the engine cooling system, which is done with a vacuum filling device. The coolant pump for heater is located on lower right side inboard of right front wheel, mounted to frame rail. When electric heat is on, you should be able to put your hand on it and feel if its pumping. Unfortunately servicing the HVAC system is not a do-it-yourself project. Do you have heat when the engine is running? I also would agree your clicking sounds like axle nuts, not brake caliper pins. The caliper pins have boots that seal in the grease and keep out moisture and dirt.
2013 White Diamond, "Hal" Black leather w/white accents, Bose