How (and what make) to add Cd changer unit to boot

Hello everyone, I am new to this site having just purchased a lovely, low mileage Y Reg, 2001 1.4 Petrol Polo, 3 door. It has the original Cassette/Radio fitted with the cd button. I have looked in the boot and to the left there are two wires and a small red plastic connector unit. The wires are brown and red. I want to add a cd changer unit. What is the red plug already there for? How do I add a 6 cd changer unit and how do I connect it up to the original radio? What sort of cd changer unit should I purchase and how much will it cost? Also if the red connector is not for a cd unit, what short or wiring/connector unit will I have to purchase.
I live in Shropshire (Oswestry) but don't know of any good car radio specialists around my area or in Wrexham or Shrewsbury that can give me advice.
Any suggestions please would be most welcome.
Thank you
Ju150