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I can post the diagram for you, but it would have to be this afternoon. I work full time away from this site, and am just about to head out the door. If I get it posted to this chat this evening, would that work for you?
Hello again. Are you able to open the thermostat box, to expose the wiring? If so, please do so, without disconnecting anything. Then, take a photo of the wiring and post it here. That way, I'll be able to direct you in where to connect your new wiring. Thank you.
The photo looks fine. I see that the supply wire feeding the thermostat is #12 wire. That could mean the circuit is on a 20 amp breaker. If so, you'll need to use 20 amp wire as well, and a 20 amp rated switch. 20 amp wire is #12.
I'm searching your photo for two wires connected to an adjustment knob assembly. Do you see anything like that?
It looks like there are 2 black wires, each attached to a metal tab near the knob.
If you follow each of those wires into their respective wire nuts, those are the 2 splices you need to add your new wire to.
One of those existing black wires will be live at all times. Please use some type of tester to find that live wire (even when the thermostat is off ).
Once the feed hot is identified, add the white from the new piece of 12-2 to that splice. Put the black from the new 12-2 in the splice with the other original black on the small metal tab by the knob.
The other end of the new section of 12-2 will go to the new switch. White wire to top terminal on switch, black wire to bottom terminal.
Complete your grounding, turn the breaker back on, and test operation. Please post the results here.