you have to setup your screenmodes first in p96 in an aga screenmode first,this may sound like a silly question but have you used p96 before?
are you trying to see a picture on the vga screen while the card is plugged in,im a bit confused.
OR are you getting a black screen in an aga screenmode while booting with the card plugged in?
i'm going to make another cup of tea,i'll be back in a bit
EDIT:i'll post a pic of what it should look like in p96mode