E46 Radio Brown positive speaker wire ?

Hello, I have the standard 6 speakers / no amp set in my car, I have a CarPlay instead of the stock head unit. Today I’ve installed some MTX TX450C on the rear shelf and when trying to find the positive wire, I have discovered that when using a multimeter. Probing the red lead onto the colored wire and the black lead onto the brown wire (which I assumed was ground) gives me a negative reading ? So I’ve plugged the brown onto the big spade on the speaker and the colored wire onto the small spade.

To my surprise the sound was clearer and there was more powerful bass compared to before when the connections were reversed.

I looked up on the TIS and to my surprise, brown is negative speaker wire. So are the bmw to iso adapters reversing polarity or I’m missing something ?