Though many do not read schematics here, I will reference colors as described in the schematic. For example, VI/GY is violet/grey line. RD/WT is red/white line. BR/BK is brown/black line. BL is blue, YL is yellow, etc.
1. Study the radio wiring with the aftermarket unit. They are pretty basic colors. Red is generally the main power wire. Yellow is generally the memory power wire (keeps power to the radio enough to keep radio stations saved, last point on a CD, settings, etc but does NOT turn the radio on). If you find after your install that the radio will turn on and work but will not keep memory, you have these two wires backward. Black is usually ground. Blue is commonly a remote wire (aftermarket amp wire). White is generally for light switch dimming control or power antennae. Other combinations of colors are used for the speakers: purple, grey, green, and white. The same colors with a black stripe on them indicates the (-) terminal for the speakers.
2. Using the schematic on diagram 6500-0 (page 99), find the VI/GY (grey stripe) wire that has a connector changing it to VI/WT (white stripe). This is main power for the radio from the battery to turn the radio on. Splice the red radio wire to the VI/WT wire, omitting the connector (C216 is a connector). You will see that the VI/GY wire in the car is looped around to itself and looks confusing. No worries, tap in to the part that you see going back into the dash. Power is done. Next, find the RD/WT (red/white stripe) wire. It will be connected to a yellow wire. IGNORE the yellow wire! Take your yellow wire from your aftermarket radio harness (unless it specifies another color for memory, sometimes blue depending on the brand) and tap into the RD/WT wire. Now you have memory power. Now there are two white wires in the factory setup. One that runs along with the antennae cable (this is the raise/lower input wire). If your radio has it you can try to splice this and see if it works. I did not do this as I have heard many on here having a separate switch for raising/lowering the antennae, so I will not go into this operation. The other white wire in the harness connects to a GY/RD wire which leads to the light switch for auto dimming. My two cars have this feature ignored. Finally, find the ground wire as BR/BK (brown/black stripe). Splice your radio's black ground wire to this. If you see the car's BR/BK wire tapped to a brown wire BR, then splice directly to that.
There! The radio itself is wired in and can function. Now, here comes the intensive part: speakers.
1. Run new equal length wire to both front speakers for BOTH front and rear speakers. Right side fr/rear speaker wires on one side, left fr/rear wires on the other. Tuck wiring behind the kick panel and in the glovebox as you want it. Run behind the kick panel carpeting as well. The wires should have atleast 4 inches left when they end in the front speaker tunnels. You will NOT have to run new wire to the back of the car. I will show you how.
2. Turn to page 100, diagram 6500-1. Wire the front speakers. Now on factory e28 speakers, the wiring for the fronts are opposite eachother on the speaker. The GY/GN (grey/green stripe) wire is for the tweeter. Remove the two front speaker wires and splice the new wires from your radio (make sure it is the harness for that speaker, remember you ran a total of 4 wires to that one speaker tunnel... 2 for it and 2 for the rears). Repeat for the other side. Your two front speakers are done, and completely operated by aftermarket wiring, alone from the car.
3. Now for the rears. This is going to sound crazy but you are in essence USING FACTORY AMPLIFIER WIRING AS REAR SPEAKER WIRING.
We aren't done yet. But By now you will see where I am going. Work with either side you want first. The right front speaker is where I will start here. According to the diagram 6500-1, you will see that a GY/BR (grey/brown stripe) wire and a GY/WT wire are there. Hmmmm you say? That goes to the amp. So guess what that means? It runs to the back of the car! Splice the 2 other wires you ran from the radio, one to the GY/BR wire, the other to the GY/WT wire. One of them looks a little confusing because of the connector BMW uses... but just splice into it and call it a day. The other side is wired the same but the colors are GY/VI (grey, violet stripe) and GY/RD (grey/red stripe).4. Now for the tie in. You have everything done up front. Fronts and rears are wired up. Now to bypass the amp and take it out of the system. Unplug the amp in the back and bring the harness out of the trunk enough for you to work comfortably. Have your schematic handy, as I used letters to match up what I needed. CUT the wires listed below.
A GY/BR (grey/brown stripe)
D GY/WT (grey/white stripe)
B GY/VI (grey/violet stripe)
C GY/RD (grey/red strip)
B BK/VI (black/violet stripe)
C BK/RD (black/red stripe)
D BK/BR (black/brown stripe)
A BK/WT (black/white stripe)
Look above and splice the wires with the same letters together. A to A, B to B, C to C, and D to D.
And boom goes the dynamite! No but seriously that is it. Clean up the wiring and tuck it all back away. Once this is done you have successfully rewired the entire speaker system using already existing wires in your car. My 88 had this wiring for 2 years, and never has it failed me. EVER. very reliable, sounds great, and make $20 selling your amp.
Also, for the sake of resale value, all of this can be put back to stock quite quickly and easily if you desire. No hacking the fader switch, and factory wiring is still in place.Now go cruise to your loud tunes. Pics to come if needed.