Welcome to the forum as Im a new member as well. I have a 2000 Millennia S with the same colorway. it seems as if we got the same deal on our cars and the interior. Mine belonged to a high school graduate, a girl at that. I've modded the intake wit short RAM, dropped it 1.4 all the way round...
As far as the radio goes you have to get the aftermarket wiring harness thats makes the impedance match up or else an aftermarket wont have sound. Thats if you have the bose system.