I didn't mount stock radio buttons anywhere - the module is too large to fit anywhere without taking too much room.
I used the long 50 pin ribbon cable that is listed in the parts list (https://www.northamericanmotoring.co...ml#post4483538).
It is connected to the factory stereo inside the dash. The other end is routed to lower center console (where the USB and AUX ports are).
Whenever I need to change something on factory radio I can connect the control panel there. I am thinking about routing ribbon cable to a glove compartment, but haven't decided yet.
So far I only needed to do this after disconnecting/reconnecting battery to set radio to AUX input and reset its clock.

 

I've been using CarPlay enabled stereo in the factory speedo for more than a year now (see my posts above) and everything worked great, up until recently.
Suddenly I started experiencing occasional hiccups during music or books playback in CarPlay. They were infrequent and very short initially so I ignored them, but lately became a real inconvenience.
The sound would cut out for a longer periods, additionally my navigation screen would not refresh properly and touches would not register.
There were no sound cutoffs in the radio and bluetooth modes however, so it led me to believe there was something wrong with my USB connection.
Reading online about similar symptom with CarPlay - most people narrowed it down to the use of cheap USB cables.
Since I have Apple USB to lightning cable going from the radio to the phone, that left only extension between radio and screen I have added using cables ordered from eBay (1.25mm 4-Pin Pitch JST PicoBlade Male to Female 4P connector).
I took one of the high speed shielded USB cables I found among my spare cables, cut off connectors on both sides and soldered it instead of using chain of prefabricated cables from eBay - not a single hiccup since! Everything works as expected.

Just figured I'd post this to save someone else time and trouble if they decide to go with similar setup.