Fitting new car stereo

[crebs]

I've been trawling the forums here looking for specifics on replacing the stock BETA car stereo in my Mk4 2001 VW Golf E. I'm pretty confident building computers, wiring electrics etc so I thought I'd have a go at fitting my own car stereo.

The stereo I am looking at is a Sony BT-2500
http://www.sony.co.uk/view/ShowProdu...+CD+Tuners#tab

The specific issues I have are:

1. The golf has effectively 6 speakers: L+R rear, L+R front, L+R front tweeters. How will this fit a stereo with 4x50w? (surely only 4 outputs)?
2. Do I need to buy a splitter of some sort or new wiring block harness? Which one? Halfords?

Any links to specific products would be really handy.

I have uploaded a JPEG of the wiring at the rear of my existing VW BETA stereo and here is a link to the wiring schema of the new stereo http://pdf.crse.com/manuals/3100200221.pdf - you can see where the confusion arises!

Thank you in advance for your replies.

Chris.