You will have to use the Nuvi screen to control the unit and to view it. The voice commands can come through the Nuvi's own little speaker or through the car's speakers by way of the Nuvi's fm transmitter. Fm systems work well enough in some areas and not good in others.

A connection through an auxiliary input is much better, but the BMW doesn't have an aux input and it seems hard to get one installed. And because of the complexity of the BMW system I don't now if it would work anyway.

Maybe someone has an aux input installed and has tried this. It would be interesting to hear about it.

So, use the Garmin and its speaker alone and use the nav screen for the computer and phone functions.

If the garmin has a video / audio output, then connect it to the BMW screen either over the AV input of the TV, or us an external video input adapter

Of course you cannot control the garmin from the BMW system, you have to continue to control the garmin on the garmin itself

i will buy this
https://buy.garmin.com/shop/shop.do?cID=164&pID=361
but - how to connect garmin, because i afraid if i use garmin i will have only garmin - no other sound or video.
is this correct?
or i can split samehow two devices?
tnx again

Yes, that GVN52 Garmin unit will work.
It ourputs composite video and analogue audio

You can connect to the TV tuner AV-IN
Then to view the nav, you go to Television, and change channel to AV

Or you can add an external video adapter to create an independent AV-IN function. Then to activate it you need to use a switch.

If you use a video-in-motion module you can then mix the nav audio with the rest of the car using say the phone interface.

Or you could modify the radio module and add an extra audio input for the nav, mixed with all other audio sources. Electronics knowledge is required.