Ok if the new unit is working and you have the code here is what you do:
Switch ignition on.
Make sure radio/cd is off.
Press and hold the AS button while you turn the set on.
CODE- - - - should appear.
Repeatedly tap button 1 to get the first digit up on the display.
Repeatedly tap button 2 to get the second digit up on the display.
Repeatedly tap button 3 to get the third digit up on the display.
Repeatedly tap button 4 to get the last digit up on the display.
Your display should now be showing the code you have for the set.
Finally press and hold the AS button until set turns on.