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I recently purchassed a early production 2012 135i that came with a Base radio setup. This means I only had the the 4" speakers in the front doors and rear deck. I picked up a set of HiFi tweeters up off ebay. They plug and play right into the existing speakers. I installed the front ones, have not had a chance to install the rear ones yet but was a huge improvement over the previous setup. Even without the "external app" totally worth the ~100$ spend to improve the system.
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