I'm not sure which plastic piece you're talking about - I think they varied a bit from year to year. I've found one of the keys to getting seats out is to leave them plugged in as long as possible so you can move them forward and backward to get access to the 4 bolts. As far as the plastic cover pieces go they just snap on, and theoretically snap off. The method I've used on a number of occasions is to wiggle and pry them back and forth with increasing pressure until they break, and then curse and throw them away. You find that you don't miss them unless you're really a nut about originality. Or another more sensible method would be to go to the dealer, see if they have one in stock, and inspect the snap points so you have a better idea of how they are attached.