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good morning I am kevin. On the inside panel of your power connection compartment, there should be a wiring diagram. If you can take a picture of that diagram, I'm sure I can help you decide which wires to connect.
Sorry it took so long, had a few calls to do this morning. Not a very good diagram I might add, but it appears that you may be able to use the chassis of the air handling unit to connect your neutral to power the thermostat. Are there any other diagrams showing the thermostat and wiring to the thermostat?
Your R wire is the hot (24V) feeding the thermostat. connect your 5th wire, I think you were saying to metal surface of inside where all the other wires from the thermostat connect. If you can follow it, there is a transformer inside the unit, one wire is showing feeding R to the thermostat, the other goes to a "ground" on the unit itself. If you can connect in the same place that should be sufficient.
Yes, that would be the easiest to determine based on the print you sent me.
your 5th wire being the "common" to the stat.