Mannekin Pis A copying Stonehenge B
Mannekin Pis uses its own tucked card.

The ability copies the effect of the wonder to their left.  This essentially means it gains the text of the other wonder stage.  That text is: "At the end of the game, the player flips face-up the card used to build this Stage. Then, the player scores 1 victory point for each card of that color present in both neighboring cities."  Reading this text in the context of Mannekin Pis' 3rd stage, "this Stage" refers to  Mannekin Pis' stage 3.

Support for this comes from the rules for the expansion:


  The stages of the targeted Wonder do not need to have been completed by their respecitve ownes for the Menneken Pis to copy them.


For this to be true, the Mannekin Pis must copy the text of the ability rather than copy any of the properties of the other player's wonder board (including what card the other player did or did not use to build that stage of their wonder).