Wiring Tip:
Hook everything up with simple finger twists. If you still have the quick disconnects that Porsche used between the speakers and the deck - keep them - they are very handy if a bit bulky.
Put cables back on battery.
Play your stereo and check that front/back/left right all work correctly (easy to get those mixed up)
Check to make sure it sounds reasonably basey. If its really flat then check that your wires are not backwards as Bob notes above.
Once you are happy solder everything up, run the wires, and install the deck/speakers.
This is a simple application of "measure it twice, cut it once"