Glad you found the parts. Behind the door pocket is a cross-over for the tweeter and the main speaker in the door. You just have to take off the door pocket and the cross-over is in a little hollow area between the pocket with the lit and the open pocket in the front. I was not sure if you are only replacing the radio or if you are restoring speakers to the original as well. If the car has aftermarket speakers, the original cross-over might be replaced as well. On the other hand, you might want to think about keeping aftermarket speakers because the original Porsche installed speaker are not nearly as good as the rest of the car
Have fun!