you have to use bmw specific stuff. BMW's use 3ohm speakers, if you use standard 4ohm they'll sound like shit.
Either rewire your whole stereo, get OEM replacements, or use BSW speakers (bmw specific upgrades)
Hot: 2001 Estoril M Roadster
ZCP 19's, ZHP knob, JL 8W3
Cold: 2002 TiAg M3 6mt
ZCP 19's supersports, ZHP knob, UUC short shift GROM, OEM 18's w310's,
http://absolutehid.ca hid kit