Go into you local SEAT dealer and give this part number:
000051389 - Radio DIN Mounting Adapter. Leon / Toledo 1999 > £ 9.99
Thats so you can fit you aftermarket headunit to the cars center console.
The standard 6 disc changer will not play mp3 format cd's as already mentioned.
Also, as already mentioned. Connect2 may do a loom adaptor so that the changer will fit an aftermarket headunit. Only ones I know of though are for Sony and Kenwood.
If you just want the headunit in and want to remove the disc changer you will also need a glovebox liner.
These are £9.99 from SEAT but I cant remember the part number. Did have one from my Leon when I fitted the changer but sold it forgetting to write down the part number.
Also. For fitting an aftermarket headunit. You may also need a wiring harness adaptor. Halfords do one for VW/Seat/Audi for £7.99. You can get them elsewhere for cheaper (eg. Ebay)
You will also need an antenna adaptor (£3 Halfords) and also an antenna amplifier (£14 from halfords)