How difficult is it to replace a 2003-2004 SL comand with one from a 2005-2008 model ?? Does it fit right into the slot (i.e. Same size width of unit)

Thanks in advance

not the same stuff. different fiber optical bus line runs thru them and integrates with the cars computers.
you can upgrade the older command unit;
https://mbworld.org/forums/sl55-amg-...eo-system.html