Did you search for the installation manual? If not search for it. It shows the thermostat connections. This wiring strip must be for internal wiring. Look around and see if you find another set of wiring terminals labeled R, G, Y, C, etc.

I don't have the manual, I just bought the house a few months ago and couldn't find it in the folder with all the manuals. I did search on SpacePak's website and couldn't find anything relevant. I sent a message to their tech support 3 weeks ago, no response.

Also, on second inspection I found there are actually 3 wires to the thermostat, not 2 - red, green and white (connected to the Y terminal of the thermostat and terminal #2 of the air-handling unit).

The question is which terminal is the common wire. Is there any way to test that like measuring voltage or resistance across terminals?

Here is the installation manual. It is showing the thermostat terminals. I am not sure what terminals you are showing in the photo.

You should read 24 V AC between the R and C terminals with a voltmeter.

Thanks, that's very helpful!

I have the same issue. What was your solution?

Okay thank you. I will try that as well. How did you decide on terminal 5?

I found the wiring diagram in the installation manual that mike_home linked (Figure 2.13). I also measured the current across R and C and it was 24V AC.

Okay awesome. I will check those out as well. Thank you!