Your best bet is to find the factory amp (they are probably right about the radio not having speaker wires) in the trunk and locate the output wires going to your speakers. Most BMWs I've worked on have the speaker wires twisted (dont untwist them). You will need to cut these wires, feed the output of the factory amp to the speaker level input of your new Alpine. The other half of the wires you just cut are all of the factory speaker location wires. This will keep you from running wires all over the car. The factory wiring is sufficient size for that amplifier. Hook the output of the Alpine to the wires going to your factory speakers. You will probably have more twisted pairs than outputs on your amp. A good way to test which location the wires are, is by using a 1.5 volt battery. When you touch/take away the battery from the wires, you will hear a little tick/pop. This tells you which twisted pair goes with a particular speaker. When you hear the front right door woofer pop, that goes to your right front output on the amplifier. If you have factory tweeters, you can tie the tweeter with its companion woofer. Hope this helps....
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