[Unusual Suspect]

Ok, first of all I would like to tell you that I've been following this forum for some time and I think it's great and people are very helpful.

Here's the deal.
I want to replace/upgrade my existing sound system by replacing the factory radio with the Sony CDX-MP80 receiver, adding the Signat Rom4 4x90w RMS Amplifier and two Pioneer 8inch speakers + a Pioneer Subwoofer. I intend to make holes for them on the plank behind the rear seats.
First, I want to remove my existing car radio. I checked out the procedure and it seems easy. But once I remove it, how can I connect the wires to my new Sony receiver since the plugs don't match? And how do I fill in the gap that will be present once I replace the double din radio with the single din? So, I want to connect the existing speakers, radio antena and a PCI (if possible) to my Sony receiver... How can I achieve that? [:I]

All kind of help is apritiated.

Tnx.