I can't follow your description of the wiring. The Nest C wire should be connected to the common side of the 24v transformer (in your schematic it is the transformer wire that connects to the common side of the indoor blower relay/external load/heat relay1/heat relay2). The other way to find it is to find the yellow wire from the transformer that finally connects to the R terminal of the thermostat; connect the Nest C wire to the other yellow transformer wire.

Got it. Thank you

Hi can you please assist im trying to connect the c-wire to the old furnace i have can you please look at this pic and see if it helps, my smart thermostat was working when i hooked it up but someohow the cwire on the hvac came loose and im trying to find out where it goes