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Old 01-18-2013, 06:47 AM
Blacklane Blacklane is offline
Officially Welcomed to the 'Fest
Location: Springfield, Ohio, USA
 
Join Date: Jun 2011
Posts: 1,026
Mein Auto: 1998 Z3 Roadster
I think you are several years late for this. The single-speaker sub boxes are no longer available and have been bought-up by users making this same retrofit. It's pretty rare to find one now.

There is not much chance of retrofitting a single speaker into the 2-speaker box. The remaining holes are too large and in the wrong places.

You can replace the two speakers. If you're using the stock amp, just use two 5 1/4" 2- or 4-ohm speakers. (The originals were about 3 ohms). If you have an aftermarket sub amp, use two 8-ohm speakers wired in parallel with reversed polarity to create a single 4-ohm channel. (You have to reverse the polarity since they are mounted back-to-back).

Or you can make a sub box. It's not that hard. I made one out of fiberglass:
http://www.bimmerfest.com/forums/sho...d.php?t=605626
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