Now that you all mention it.  The BMW MC list (Internet BMW Riders) posted a 
while back that the accessory plug you mention was also carried by your local
John Deere Farm Implement folks.  Sounds odd, I know.  I don't have the P/N
but you could also check with yflbmwmcshop for their price.  I seem to remember
around 7$US.  Try Kari Prager's California BMW in Mountain View CA. (Area
Code is 415, I don't know the number)

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