LOCATION OF A/C LOW PRESSURE SWITCH
 

Have a 2008 Expedition XLT with Rear Air and need to find the location of the A/C Low Pressure switch which appears to not be working. Can get compressor to engage by putting 12vdc on Purple wire at compressor and then unit works fine until it sits for a few hours. Have taken to dealer (45 miles drive) but it is working when I get there. Accumulator/Low Pressure Switch is not at the firewall like it is on my 2008 F-150. Anyone know were they hid it??

That sounds more like a clutch gap issue than the pressure switch.
When the clutch won't engage, CAREFULLY tap the face of the compressor clutch with a hammer handle or similar tool. If it engages, the clutch gap is too wide.
See the FAQ in the AC section for the steps to adjust the clutch gap.

Do not get the 12 volts to the clutch so cannot be the clutch. The 12 volts to the purple wire on the input connector comes thru the Low Pressure switch.

Yes, you can often get the compressor to engage by applying 12v directly to the clutch coil when the gap is too wide. Did you check for voltage at the coil with a meter or test lamp?
Do the tap test first before you condemn the switch. Those switches usually fail totally open or go out of range.

Did the voltage test with a volt meter and also made sure I have a ground to the Black/Green Stripe wire to the clutch and tapped the clutch hub before I started with the volt meter. Since I had no voltage to the clutch connection terminal, I am sure its not the clutch. They must have the Low Pressure switch back at the rear unit somewhere........

The switch will be under the hood and plumbed into the Low side of the front AC.
Follow the larger (suction) line from the evaporator, to the accumulator, and back to the compressor. It will be there somewhere. It is usually screwed onto a fitting on the accumulator, but I'm not familiar with part placement on your particular vehicle.