Getting the needs
You need the following:
Scrapyard:
Fiat 100W (active) subwoofer

Subwoofer mounting bracket

Subwoofer connector plug (best with some wire left)

Mounting:
Plastic rawlplugs

Some plastic / metal rings to level the rawlplugs with the cars metal
Electrical:
About 7 meters wire (buy 4 meter 2MM, 3 meter 1 MM).
14x - In-line connectors (clamp on ones)

1x - Fusebox splitter plug (for mini fuses)

Next: Preparing the car!