Edit...if this was a V6, I'd say "Heater lock out switch". Now that I see it's a v8, I'm probably wrong but don't want to delete my post...maybe the A/C pressure switch...
Heater lockout...
When your eatc is on auto, and it cold outside, you expect heat from the vents...but when the engine is cold, nothing but cold air would come out. This little switch makes sure the water in the heater core is hot before kicking the blower motor on.
This can be observed by driving the car and turning up the temp on the eatc...the blower motor does not start right away. It comes on as soon as the engine warms up (more accurately...as soon as the coolant in that pipe is hot).