Try sanding it off with a 3 inch disk sander and a real coarse grit like 24 just dont sand thru the other side. I herd options when ordering that you could get extra large excess carpet so you could trim to your desire but I think that option is only for the front.
Update to my install....after MUCH cursing, I finally got the rear carpet installed. I contacted Stock Interiors (who I bought the carpet from) even though when it showed up it was in an ACC box. They told me that it would not be a problem to install with the backing on......I say...NO WAY, NO HOW. The Mass backing is just too thick to allow the carpet to roll over the pinch welded portion of the floor where the sill plate has to lock it down. I finally took a Dremel with a drum sander and ground all of the Mass backing off at the area where the carpet wraps onto the sill. There is no way it could/would work otherwise. Lesson learned....DO NOT order the Mass Backing option, even though they tell you it will fit better and lay better. Hopefully tonight I can get the front carpet in with less effort!
