I've already taken others advice and disconnected them but it's bugging me with what's going on. Before unhooking the grid heaters they would cycle when it was cold enough for them to come on but for some reason they would cycle constantly instead of 30 seconds on/off/on. They would also still be humming when I would turn the key off. So, what's going on with them? I doubt I'll hook them back up but this is bugging me

 

If both heaters remain on with key off, either both coil drive wires to the PCM are shorted to ground (unlikely), or the PCM is bad.

I'm assuming you are saying the heaters remain on constantly. The word "cycle" means turning on and off in a rhythmic pattern.

 

If it's cold enough for them to come on, and depending on how cold it is, then you will see them cycle for up to several minutes. And not always on a regular, and predictable cycle either.

I just fired my truck up this evening at work, and they were cycling for almost 5 minutes.

 

They stay on constantly rather than cycling on and off. Also, if I just turn the key on and off they start running when I turn the key on but they remain running even after I turn the key off.

 

Sounds like you have something chaffed, they work by applying ground from the PCM, so start checking the trigger wire all the way to the PCM.

If the wire wrings out OK, somehow the PCM has shorted to ground on that circuit.