Getting a common wire from air-handling unit to thermostat?

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I'm trying to replace the existing old-school thermostat with a Nest, but there are only two wires between the air-handling unit (

SpacePak ESP-3642D) and the thermostat (green and red, no common wire), and the Nest wouldn't power up.

First off, is it reasonable to believe the problem is the common wire, as my google searches indicate?


If so, I have some 18-gauge wire that I can easily get between the air-handling unit and the thermostat, but when I opened it up to see where to connect the common wire on the AC, I found no indication or labeling on the terminals other than numbers (see pictures).

Can anyone help me figure out where to connect the common wire on the air-handling unit?

q getting a common wire from air handling unit to thermostat
q getting a common wire from air handling unit to thermostat
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