Dont connect the common wire.
Use what is called bell wire or thermostat wire.
That Thermostat is LOW voltage. IT does not have a ground wire. There is no need for a ground. it only opperates on 24 volts. there may be a green wire but it does not mean it is a ground
follow the lower radiator hose until you get to where it connects to the thermostat housing practically on the engine. There will be a few wire connectors and three-four nuts holding it on.
Do not do this.
Thermostat wiring can be hooked up by following the colors of the wires. Each thermostat comes with color coded wires that are the same colors as the thermostat already in place in the home. Hook each wire to its colored wire.
If your thermostat is designed to switch the fan on and off at say 120 volts, then it acts just like a switch. This means that you insert the two wires from the thermostat in series with the black wire going to the fan. So let's say you have a supply of power for fan that has black, white and bare wire ground. You connect the white to white on fan and ground to ground. You then connect one wire of the thermostat switch to supply black and one to fan black wire. If you are unsure of the type of thermostat or anything else get an electrician. This is just one possible solution given the information provided.
mounted right next to the thermostat , has one wire going to it.
If you have a heat pump it will be on the thermostat, if the thermostat for some reason has been changed to manually wire up emergency heat , remove the thermostat after you kill the power and connect the red wire too the green and white wire then turn the power on, you will have heat.
Yes, using DPDT relays between the thermostat and the two furnaces.
Red
the thermostat has a black(line) wire to it, and a red wire going to it. the red wire then connects to the neutral wire. the black and red are like a leg switch.
In the heater you will have two wires. You should then have 2 supply wires from the panel, and 2 wires from the thermostat. The neutral (white) supply wire should go to one of the wires on the heater. The hot (black) supply wire should connect to one wire from the thermostat. The other wire from the thermostat will connect to the other wire from the heater.