See attached link:-
http://www.wiganlatics.co.uk/javaImages/1a/3/0,,10429~6751002,00.jpg
Car Park 1 is that referred to above, being for away fans.
I recommend, whilst it's further mileage wise, getting off at J27 and travelling south through Standish rather than getting off at J25. Did this for the night game last season and got there, and away, much quicker than on my previous visits. The traffic seems to get very congested on the retail park side of the ground.