No power to Thermostat

I have a problem that's been plaguing me for the last couple of weeks and asking for some help from this forum.
Located in Dallas area, and with dual heat pump systems, there's no power to the thermostat on one side of the house.
The systems are identical Goodman compressors outside, Goodman air handlers in the attic and White-rodgers (Honeywell) digital thermostats.
One day, the one side lost power and the other side remains functional.
Being that they are identical systems, I thought this could be troubleshot and repaired relatively easy.
Batteries on thermostat have been changed and verified good.
To date, virtually all parts have been swapped from one side to the other with no positive results as the symptoms have not changed on one side, yet the other side remains fully functional.
Parts swapped :
Thermostat
Circuit board (including fuse and fan relay)
Transformer
Fan time-delay relay
Contactor (on compressor, and press-to-test good)
Capacitor (on compressor)
And whatever other part I may have forgotten to mention has been swapped.

Probably a shorter list would be what I haven't swapped which would be:
Actual (squirrel cage) fan
Compressor itself

Circuit breaker (located NEXT TO) airhandler shows good voltage.
Circuit breaker ON AIR HANDLER shows good voltage.

Power to everything is good, power coming out of everything is good.
Everything that needs to be ohmed out checks good.
All this verified on both units.
But the problem remains that there's no power to the thermostat.

Google searches mentioned somthing thing about a float switch but I can't find anything on the air handler that looks like a float switch, let alone any wiring that traces back through the system from where a float switch would logically be located.
I think I've exhausted all logic and asking the experts on the forum to guide me to one of those weird instances or something I've completely overlooked.
Thanks in advance as I'm at my wit's end.