2004 GMC Envoy XUV Rear Glass

Trying to find out how to make the rear glass go down on an 04 XUV.  This is the diagram that has been posted on here already and it appears you can get the rear glass to retract via the overhead console with a resistance based pulse.  Anyone know what resistance this requires?

There is also a button in the very rear for the "Emergency Express Down" feature. I can't seem to find it in the diagram, but it's defiantly there in the vehicle.  It only requires a single pulse to activate, and the ignition does not have to be on (accessory powered up) like it does for the button in the overhead console to work.  Anyone used that as a source yet?  Is that also resistance based? 

Also, I've always wanted to ask this.  If a circuit in a vehicle is resistance based, how do you test to find out which value you need to duplicate that operation?  There must be a way to test.  I'm sure companies like directechs and wiremagic don't just "guess" until they get it right.