Test them to find out.
If you take a small battery (AA) and soder a short piece of wire to each end you have whats called a speaker bumper.
In bmw's the speaker wires are twisted together.
Touch the wires from the bumper to the factory wires.
The speaker that that is connected to the wires you touched will cause the speaker to make a small noise.( you might have to get someone to sit up front to help hear which ones.
Once you have found the right speakers you will have to determine which is positive and negative.(easy easy)
In bmws the negatives are brown or have a thin brown stripe.
Hope this helps