Crutchfield says none of their 3.5" speakers will fit in the GN's dash. What speaker have you used, and how tough were they to get in there? Here's the list they have:
http://www.crutchfield.com/S-05Gn0lTd2o7/cgi-bin/ProdGroup.asp?g=51100&avf=Y&skipvs=T
http://www.crutchfield.com/S-05Gn0lTd2o7/cgi-bin/ProdGroup.asp?g=51100&avf=Y&skipvs=T