no biggie.
before you start... have radio code ready.
Identify positive and negative terminals and make sure that the positive wire goes to the + terminal; and the negative wire to the - terminal.
after replacement and you are done.... alarm might initialize and start blaring. DO NOT BE ALARMED. calmy close doors, hood and trunk, and lock. Then open. In some cases, you might need to turn IGN on before the system resets.
If this is your first time to do this, I suggest you do this in a place away from a residential area. If you don't get it correct the first time, It can get annoying. Don't lose your calm... you'll be fine
IMPT: make sure the batteries of the keyless entry are "GOOD"... (this is where the horror starts) and peoples' minds get thrown up in the air. but with a 2004, keyless batts should still be fine
part of the horror/s complicate when overzealous owners attempt to ADD other aftermarket anti-theft devices to a bimmer. Bimmer anti-theft circuits just do not work well with aftermarket anti-theft systems. if you car has aftermarket anti-theft. WATCH OUT!