You probably didn't get an answer because this is a tough one! MB/Becker really loves off size speakers with non-standard impedence that nobody makes other than themselves! If the speaker is EXACTLY 6" and designed to work with a Becker amp, then about your only choice that I know about (without cutting or custom work) is to order an MB speaker from your dealer, or pull one out of a parts car at a salvage yard. Either way, there may be an audible difference between your remaining original speaker and a new/used one due to aging factors.
Depending on the price from the dealer for a replacement speaker, there is another option. Ignore the door speakers, and buy one of the self amplified Blaupunkt CMS-265 mid/subs to put under your seat. (Does your Aiwa have sub out jacks? Low frequency filters for the front and rear?). I don't know how your Aiwa is hooked up, but if you are running the existing 6 speakers off the 4 speaker outputs on the head unit then bypassing the door speakers will give you a bit more power to the remaining 4, especially if you can hook up to the sub out, and cut the lows to the stock speakers. http://www.continentalimports.com/bl_speak_spec.html
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