radio/security system question on 01 tahoe

[60ratrod]

so the tahoe i picked up friday has a few issues, most of which i got figured out. the first is if i leave this thing locked long enough when i open it with the one and only key(the dealer didn't have any spares or key fobs) the alarm goes off and i gotta loosen the negative side of the battery until it stops. is there a way to stop this? also the factory cd player doesn't work real well and was wondering how hard is it to convert this thing to an aftermarket deck? someone told me that at about the 01 model year level they started integrating part of the computer system into the radio. so with that being said will i have to get some kind of adapter or something?