kpaske,

Check the pulley on the door popper system. Mine slid down a bit after initial installation, and I started having the SAME problem - the door not opening unless I held the popper button in for a couple of seconds.

Govehix, the door will be able to carry the weight of the subs, but the plastic interior piece (if that's what you were thinking of mounting the sub to) will not, and will provide no accoustics for the speaker, either. You need to build a sub box that will match the shape of the door and mount the box to it nice and sturdy.