Can anyone explain to me the operation of the pull down mechanism? What is the activating signal that initiates the motor to engage the lid and pull down. It looks like a solenoid with a plunger, but what activates that?

There is a switch on the right hand side of the plunger that Usually goes bad. I just replaced mine, got it at Cadillac for about $22.

If you search these forums there is threads that will explain how to replace it.