MihaiFlorin And what's frustrating me most is that I solved it a week ago by changing the Google app settings back and forth for like an hour. Now, I tried to redo it but can't get it to work on SWC. Please update if you do.
More details:
In Android settings --> Apps --> Google
I clicked on the 3 dots on the right side to "uninstall updates". Afterwards Google Assistant was "working on this device". Then I had to activate GA, update Google app again (70 MB approx. from Playstore).
I tried around with Gemini and deleted it again. No clue, what to do next.
Turn on OK Google
wybitny2 Would you please let us know what you did to get GA on the SWC?
wybitny2 I would like to know too. For me Hey Google works. But the button on the wheel opens Google search.
15 days later
- Edited
Good news for those who were not able to solve this issue of the steering wheel button triggering Google Voice Search instead of the much more useful Google Assistant. I found it and I can now open my garage door by just pressing the steering wheel button and gently asking Google Assistant to do it for me 
DUDU6 on Mekede M6 Plus 3D (I guess it should be the same for everyone though)
Settings
System
Android Settings
Box Set
Voice Package Set
Select "Assistant" and enjoy (or any other app that you may want to open with the steering wheel button)
EDIT: Now I would love to kill it by pressing the steering wheel button again instead of having to touch the screen out of the assistant window to close it. And would also be great to activate a dark theme but I can't find it anywhere for this app. (this last one was just a matter of switching on the "Sync with Android Global Day and Night Mode" in Home > Day/Night. It is now light mode or dark mode depending on the car lights)
13 days later
Very good piece of advice. Thanks!
Recently my Toppal Assistant stopped reacting and I could not reactivate it again so I deleted it and right after that Google Assistant would react to pressing the SWC "Mode" button. That's very much appreciated.
I am also interested in how to conveniently close the sill open Google Assistant window without touching the screen.
Also, closing apps by using Google Assistant like in "Close Youtube", is not working (but was previously with Toppal).
Moreover, I was trying to voice activate it and sometimes it did react. Totally random it seems. But anyway, if the steering wheel button works, it's fine.
- Edited
suicidalguitar
Edit
I have had a look onto the settings you mentioned before you posted when I was trying to hook up Google Assistant to the SWC button. Back then, Google Assistant did not show up there, so it might be an update that did the trick.
@YAOXILONG
<FEATURE REQUEST>
Would it be possible to close the opened Google Assistant window by clicking again on the SWC button (as to avoid having to touch the screen every time)?
Would be cool if Assistant could make calls. But that is not working.
JhonnyD They just implemented the SWC support for Google Assistant. With patience, I bet we'll have calls through GA and much more. Everything takes time.
5 months later
When does GA will work?
I,am very disappointed that it takes so long 
7 days later
@DUDU-Meng
<FEATURE REQUEST for future updates>
Would it be possible to close the opened Google Assistant window by clicking again on the SWC button (as to avoid having to touch the screen every time)?
Could you also increase GA functionality to close apps? Make calls via the Phone bluetooth app?
- Edited
HondaCivicDriver For me in 90% of the cases the GA window closes by itself. I don't know why yours isn't. Have you tried pressing the MODE key? I have nothing associated with MODE key in SWC settings and when I press it has the "get back home/desktop" function now. By default it is associated to the multimedia app. But If you delete multimedia from SWC MODE key settings it will behave like a "go back to desktop" command.
5 days later
MihaiFlorin
well, my mode key is linked to various apps, so i can`t do as you propose. I want to keep the mode key linked to apps, as this is of higher priority than closing GA.
Do you utilise all 5 mode app choices by any chance?
If not, set Mode App 1 to empty, and put your apps on 2, 3, 4 and 5.
I believe the first press will always be Home regardless what app is open, then a second press will start to cycle through your set apps.
@CHR12345 I will try as you say.
Besides, I was quite amazed how well Gemini (not Goggle Assistant) is working as well. It seems it was recently updated by Google and is now more like an AI model. It can even use Google Maps to navigate me to any place by voice control. It can open apps, but not close them. However, it cannot open the Bluetooth Phone app. That's a pity...
Gemini / GA immediately reacts to my voice command - unfortunately the SWC activation for Gemini opens a window which i need to click to start it unlinke the "hey Google" voice command which directly starts Gemini.