While I cant speak from personal knowledge, my guess is this:
The movement of the pin is the key. It probably doesn't ACTUALLY matter which pin you move to the new location, only that you have A pin there (it just so happens that the MUTE pin is nearby and not very useful).
As to the specialness of the new location, I suspect it is placed there as 1 of 2 things:
1) A developer debugging tool, that is left in the final design.
2) An intentional 'for the customer' bypass for use in non-car application (e.g. in the back seat of a motorhome, or in a boat).
I would bet dollars to