Wiring is not there unless it came with it, a cheaper way to do it is
to get a good 4 channel amp. Wire in the RCA converters to the back of
your deck (about $8 ea. at Radio shack) pick up a set of
components(mids and highs) for your front doors, the benefit of buying
them together is that you usually get the crossover with the set. Then
just plug it all in. I know that people will disagree with the RCA
convertors, but the stock deck is fine for pushing signal. and the
bonus is that it keeps people from breaking into your car and stealing
your stuff... Think about it, are thieves more likely to break your
window for the stock deck out of an 89 325 or for some nice aftermarket
unit. If you get really ambitious the decks out of the e39s fit nicely
with minimal modification and then you get all the cd changer control
and the newer, nicer set up.