how do i replace the in car tape player for a cd player?

i have been to halfords time after time and there not able to help me, i want to replace the in car cassette player for a cd player i have a fiat punto active 05 plate. i have removed the facer and cassete/radio and purchased what i was told was all i needed the wires for my sony cd player (as it didnt come with the wires as got it from a friend, it does work) i also bought a harness adapter which i was told is essential to change over from tape to cd. however when we plugged the pluggs in there was nothing at all, looked in the plugs and the red wire which is the main power supply the clip on the bit it plugs in to had no metal in it for it to touch so obviously was why it was not working. went back to the store they told us to try with out the harness adapter, this time the main power suply would be metal to metal and when we pluged it all in it made a noise and the cd player would take the cd and eject it but it had a mind of its own u didnt have to press the button and if u did you had to move the cd player at different angles.. :/ there was no light up on the player either we went back and they didnt no all i can think is they sold us the wrong adapter as still not all the wires are metal to metal... but they ran my number plate through there system which says we have the right parts ive spent a fortune on all the parts can any one help it should be such a job ive bought the magnetic keys to remove the facia and a new facia and also an areal adapter as well im driving round with a gap in my dash and no music now pleasee help ?? thanx in advance