Some have reported that one of the plugs was taped to a wire bundle under the plastic trim going up the side of the A pillar. Many have reported that the plug was hidden better than Al Capone's vault.
I thought 98's had both plugs installed, but who knows.
Even if there was an aftermarket unit in the car before, there wouldn't be any good reason to use or remove the cable that goes from the HU to the floor.
There used to be a link on here somewhere that showed how to connect a changer even if one of the factory plugs was missing, but I can't seem to find it