installing a headunit in a 1989-91 325i! need help now

ok well my friend ordered a alpine head unit for his 1989 or 91 or somethin 325i

how do they install? ive only done my car (a lumina lol) and ya i needed a mounting kit for it and i needed to buy a harness to solder all the wires and stuff

my dad had to buy a mounting kit for my sisters 200sx too, so how do bmw's work?

do they need a mounting kit also? what about a wiring harness?

how do u pull out the stock radio?

please help i need to know soon cuz hes probly getting it tomorow

ok ya we got his radio last night and we took out the stock one yesterday but the mounting bracket that came with the head unit (he ordered from crutchfield so it was free)

but ya it has like 15 pins or so the bracket in the car has 8 and its not even the same size and im pretty sure that they are for speakers (those 8 pins) so i dunno whats up with that, i think they are sending him a new bracket