There is no easy way to add radar detector output to the factory Bluetooth radio setup. Some guys use a bright LED to flash when the radar detector goes off, and just give up on audio warnings.

Baehr makes an intercom kit for the K1600. You still have to tap into the existing speaker wires, but they include a nice mounting shelf.

That would allow you to use the same helmets as on your LT.

 

Baehr make a K1600 box that is pretty much the same as the earlier 1200RT and 1200GT boxes - rider, pillion and three input sources. The difference is that it needs to be spliced into the OEM system and it needs to be connected to a switched power source. If you use a Nav IV or V, the Baehr also requires the Baehr bluetooth box as well.

The loom that is spliced into the OEM system has a toggle switch so that you can switch the output from helmet/intercom to the bikes speakers. The setup makes full use of the BMW on board system for radio/music and GPS.

As mine is set up, the Nav IV connects to the Baehr Blue tooth box which takes up one port in the intercom box. Phone connects to the GPS via bluetooth and feeds into the system the same way. The sound system connects to a second port. I power the systems from the socket on the right side fairing. I used posilock connectors and or splices when I fitted the system and it hasn't missed a beat.

 

I use a set of red led's mounted on the flat ledge between the GPS and the curve of the tach. It catches my eye every time. Radar Detector Visual Alert for Motorcycles
The wafer cut is the one you want. I put all the stuff behind the right speaker for a very clean install. I did this first thinking I can add sound later. I no longer need sound.