How do I set up my Airport Extreme only for 'Back to My Mac' while retaining my current wireless internet connection?

BTMM seems to fail on the AE where it is bridged in the network.. therefore if you are running another modem router.. and wish to maintain its wireless you might have severe issues getting BTMM to work with the AE..

For a client in the network.. that is dependent on the upnp or nat-pmp being available in the main router..

But for the AE to work test it in bridge.. if it fails, test again by manually forwarding the ports required in the main router to the AE.

If you have issues using Back to My Mac - Apple Support