I am trying to add an Ecobee smart thermostat to an older Intertherm furnace and A/C. I have extra wires running to my thermostat, but can't figure out what to connect to in the furnace wiring. Any help would be greatly appreciated.
Use the existing black wire
From the wiring diagram for your unit, and the way your A/C appears to be wired, the light grey wire your unit provides for thermostat connection is the C wire. This, in turn, is connected to the black wire in your thermostat cable. So, simply use the black wire coming out of the wall at your thermostat as your C wire, and enjoy!