Buy the harness. Install your head unit, nuff said.
Honestly, your sound should improve from running the 4x50w amp in your head unit, your amp will step down the wattage on its own, but for signal integrity, the more power the better. The only caveat bieng the stock speakers, and whether you decide to leave them or upgrade them.
If you plan on aftermarket speakers, or a full sound system, however, the stock amp in that car is too low of a wattage to run aftermarket speakers with even a hint of clarity or clean sound in general. Here's the order I'd reccomend for upgrading your complete sound system.
1. 4 Channel amp
2. Rewire (Not difficult at all, takes about an hour to get everything nice and hidden)
3. Mono/2 channel bridged amp
4. Subs, (Don't go too crazy, 2 15" component subs are overkill, you do drive a BMW, you don't need earth shattering bumper loosening basslines)
4 channel amp and some decent two way/three way speakers, for the e34, they're some odd size like 5 1/4 if I remember correctly, not sure about the e39, if they're 6" or above, you can probably ignore the rest of this unless you're an audiophile. You'll want subs eventually, since with such a small speaker size (5 1/4 as oppoosed to 6x9) , you won't really get all that great of a boom from them. You will, however, be happy with the results for about 6 months to a year, so you'll have some time to think about it.
when you install that amp, ignore the stock wiring, its too small of a guage to be useful, and you'll spend DAYS trying to figure out the stock wiring harness and what goes where. Just get some decent speaker wire, and hide it behind some panels, its not hard with all the trim in the doorsills to sneak it under. The location of the battery is also sorta nice for this kinda setup. You won't have to worry too much about running power wires, just punch em through to the trunk through the openings BMW so graciously provides. Worst case, run them up the seat through the ski bag opening, its not hard to punch through and is hidden by your rear folding armrest.
All in all, for just adding a head unit, the BMW sound system isn't that bad. But as with all things car/car audio related, you sorta get stricken with some sort of fever that makes you want more and more and more.
Last edited by d3ity; 05-30-2006 at 02:15 AM.