Mine are not working either. I know this thread is is for the part number, but I figure I'll go ahead and throw another question into the pot.
I read on another forum (for expeditions) that the sensor should make a ticking sound when in reverse with key on (think very faint stun gun sound). I turned key to run, put truck in reverse and set brake. Hopped out and put my ear next to each of the 4 sensors and they all had an audible ticking noise.
I read on the same forum that if any part of the Park Assist system fails, the whole thing stops working, and this includes the mini speaker and the control module.
So my question is, Does anyone know where to find the control module or speaker for the park assist system? I have been searching for a couple weeks and found nothing.
And to the OP, I think the part number you need is 4F23-15K859-AA