You should find a wiring diagram on the top of the old unit, or failing that they will use standard coloured wires... (hopefully, but it's not always the case).

If all else fails...

http://www.seatcupra.net/forums/showthread.php?t=107740

If possible I would rig up an extra plug in the line. ie:

Stock wires -|PLUG|- 10cm wires -|NEW STEREO PLUG|-

The idea is now you can always keep the same plug on the stock wires and pull out the little extension bit when (if) you change head units and solder the new one together outside the car. Much easier.

Sure I haven't done this to my car... but I will next time round.