Sounds like you're checking the right things.  Some thoughts to items in both of your posts:

• If I remember correctly, the switch is NC (Normally Closed) and opens when you turn the wheel.  This is a safety deal to prevent you from engaging if the cable isn't plugged in correctly.  That's why AMC4Life had you unplug the cable and see if it still engaged.
• As far as measuring the two pins on the harness when it's unplugged from the switch... Yes one should have voltage.  The other should read around 0 volts but should not actually be tied to ground.  Those aren't quite the same thing.  If you can measure resistance, there should be resistance between that other wire and ground.  I'm not sure how much, but probably 100 or 1000s of Ohm.  It is basically going to circuitry to read the voltage, so it wouldn't be a direct connection to ground. 
• Measuring resistance across those same pins on the pressure switch makes sense.  Don't know how familiar you are with multimeters, but some of them just show 0.00 when there is an open circuit (no continuity).  So be careful with how you're interpreting that.
• Only 1.0 Ohm doesn't really count as 'open' or actuating the switch to me.  That should be much different.  Does your multimeter auto scale?  Is it kilo or mega ohm by chance?  Any reading of true 0.00 or 1.00 is suspect the multimeter is telling you something else besides an actual resistance measurement.  Likely an open or out of current range reading.
• Any chance you've got items to extend that connector up into the cab?  You could then add or remove a jumper between those two pins to figure out how the system is expecting the pressure switch to work.
• If your pressure switch does only toggle between 1 and 0 ohms (or some other really low value), then yes it seems you probably do have a bad switch.
• If you can probe the wire that normally doesn't have voltage while everything is connected, I'd expect it to have 12V while not steering and 0V while steering.