Hi Mate
I bought one off ebay for £40 and fitted it myself, took about 15 mins all in.
You just need to pull out the head unit and run the cable from it down the side of the centre console to under the seat.
I was quoted £60 for halfords to do it too!
The model you want is the VDO daytona CH600 if i remember correctly, make sure it comes with the cable though, cause they are £85 to buy apparently.
Cheers
Kenny