well, the final writeup on my sound system!
i have an 88 e28 528e

Parts and prices

Deck: Premier DEH-P860MP $360
Front Speakers: MB Quart Premium Component PCD 210 (1Ħħ Tweet / 4Ħħ Mid / Crossovers) $109.95
Rear Speakers: MB Quart Premium Component PCD 210 (1Ħħ Tweet / 4Ħħ Mid / Crossovers) $109.95
Subwoofer & Amplifier: 15Ħħ RF Punch XL / Directed 500W 4 Channel $150
Monster Amp Wiring Kit 4 Gauge $50
Monster Cable Micro XLN 4 Channel RCA $40

Total: ~$820 (i think it was a little more due to shipping)

here's the good stuff!

deck:





front mid installed:



front tweeter and wiring (had to slice the wire protector and tape it up with electrical tape, only thing i could think of):





rear mid + tweet:



trunk (yes.... 15" sub = too damn big, but too late to turn back):


sexy looking crossovers:



that's it! completed system....