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)
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2002 TiAg M3 6mt
deoranged, dechromed, led tails, ZHP knob, UUC SS v3, GROM, OEM 220M 19's, Michelin Supersports
2006 YZF-R6, bunch of go-fast stuff