Oh right. well ive got a after market head unit: pioneer Avh p5900 dvd, but its a flip out screen one. but was in my old car. but doesn't really look very good in my clio an when you switch the engine off all power is cut to the head unit which means the screen stays half out which is really annoying. an means i have to click a button on it to close it every time before i turn the engine off which is too much hassle for me haha.
The problem with standard head unit's is their lack of RCA outputs!
What you'd need is a high-to-low converter, which converts speaker outputs to RCA outputs They're available on ebay for next to nothing.
You'll still need a small amp to power the sub, as the headunit can't power it, but the high/low converter will allow the connection between amp and h/u using RCA's