If you just want to listen to a GPS device bluetoothed directly to your helmet, as opposed to going via the bike's inbuilt comm's system, you do not need the BMW bluetooth gear. I use a Scala, in a Shoei Multitech with no problem. Why should there be aproblem, as it's just a Scala talking to a GPS device. That the GPS device is powered by a 1600's electrical system is incidental.
BUT....
If you want to route the GPS device through the 1600's inbuilt comms system, then out to your helmet via bluetooth....Or of you want to listen to the 1600's radio, say..... Then you will need the BMW bluetooth package. I have a BMW system helmet with the BMW bluetooth too and it works fine. From its operation, I suspect it's just a tweaked Scala system, nothing more.
Why doesn't the Scala work with the full 1600 BMW comms?
I can only assume the bog standard Scala operates on a slightly different version of Bluetooth to BMW's. Scala may (or may not) bring out a suitable patch or BMW might. If you contact Scala (Slimbo had the boss man on his radio show, do a search, who may be able to put you in touch) they may well tell you.
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In passing, for what it's worth (and I know it's not what you asked) the Garmin 660 will fit into the 1600's cradle and work.... but you will lose some of the functions available from the iDrive wheel thing. Of course you can still touch the 660's screen, so it may be no big deal. I use a BMW Nav IV but have used a 660, too. I like the iDrive wheel whatsit, I must say.
PS Judge is right about the fitting. It's a pig..... But, I get the impression that once you have done it then it's quite easy to do it again. The dealers do not bat an eyelid but it had me stumped, I must say.