dash speaker removal

I have a 1991 Fleetwood Brougham, and I need to change out the speakers. I know how to remove the rear 6*9's, but the front dash 4*6's are giving me trouble. What is exactly the right way to remove the dash top so I can get to the speakers. I have a chilton's manuel, but it's kind of vauge in the procedure. I got all the screws (4 of them) reomved, but the dash top still will not budge. I'm not familier with it, so I don't want to force it unless it's what I have to do (a replacment dash top will cost a lot to replace).  

I UNDERSTAND COMPLETLY, U HAVE TO GO BEHIND THE AC VENTS IN THE MIDDLT OF THE DASH AND DISCONNECT THE DASH FROM THERE. USE A FLASHLIGHT TO LOOK IN THERE TO FIND THE CONNECTOR. I HAVE REPLACED MINE ON A 80 FLEETWOOD AND 81 FLEETWOOD. I WAS A ROOKIE ON THE 80 THEREFORE I KINDA TORE THE DASH UP BUT ON MY 81 I HAD A LIL HELP FROM A INSTALLER AND WE GOT IT OFF. GOOD LUCK BRO...