every '05 limited I've ever seen had the RB1 and infinity from the factory...
since the problem supposedly appeared after someone 'played' with the radio, you can try to pull the IOD fuse for a few minutes (10-15) and see if anything changes (aside from resetting the clock and possibly presets..)
[that would be the easiest starting point..]
if you get around to pulling the top dash trim, you will have access to the front speakers (they should have amps on them..if they're infinity)
My '05 Limited w/sunroof does not have the RB1 deck; it has the RAZ w/six disc DVD changer at the bottom of the console. Go figure.