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Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to betaken from straight on, in very good light.
You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here. ,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If there is no paper clip-icon, look for the '+attach files'link at left of your send button.
I can mark it up with trouble shooting instructions.
Tell me in detail why you need a fan relay, the newer unit have the fan relays built into the printed circuit card, the very old units needed a fan relay added to the furnace if you fit air conditioning into the system.
We can go from there.