1)Yes it has only AiNet AUX input not RCA.BTW the deck can play MP3.
2)I haven tried using the password because frankly i don't see a need to.Incase you do want to use the password, refer to page 32 and page 6 of your manual.Please READ your MANUAL.
A word of advise, if you intend to hook up multiple(morethan2-3) externals amps to your 7998R, please ask your installer to install a relay to the deck remote connection and run the remote cables from the relay itself, remember to have him put a 10-20A fuse in it.