Is it possible to add a "C" wire to my electric heater?
On the schematic, the BL wire in the section labeled "Low Voltage Field Connection" near the bottom of the diagram. That's the C wire. 


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It appears as though the "C" wire is bonded to the chassis, so you should be able to connect the C wire from the thermostat to the chassis as well. 

In the last photo, if you follow the yellow wire from the thermostat cable. It connects to a yellow wire from another cable, which I would assume leads to your A/C system.   The blue wire from the A/C cable is attached to another blue wire, that blue wire ties in with all the other "C" wires.   So you could connect the C wire from the thermostat, in with those blue wires as well. 


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