What
jrwalte stated above is true and would apply in your setup. With Apple CarPlay, the digital audio from your iPhone would be sent to your OEM head unit and it would act as the DAC, and then send the output over your OEM stereo's ANALOG outputs to your DSP via however you have it connected, i.e. high-level speaker outputs from the OEM head unit or Line Level (RCA) into your DSP. So you would lose that Direct Digital connection that you currently have with your setup.
Yes, you would have integrated Apple/Google/Waze NAV on your OEM display as well as music playing/control, including Siri Voice Commands and your OEM SWC and Touchscreen control. Note that only a select group of music playing apps are compatible with Apple CarPlay, and the same applies to Android Auto. But most of the more popular music apps are supported.
For $750 for the "Nifty City" product, I would definitely give it a pass. IMO it is not worth that much money just to gain Apple CarPlay or Android Auto!
If you already have WIRED CarPlay but want the convenience of WIRELESS Apple CarPlay through your OEM head unit, check out the new
ZZ2AIR-CP WIRELESS Apple CarPlay/Android Auto interface at
https://zz-2.com/products/zzair-cp
YouTube product promo video...
https://youtu.be/A1OokA5V0oU
If your OEM head unit doesn't have Apple CarPlay at all,
ZZ-2 also make CarPlay/Android Auto interfaces for OEM systems and aftermarket stereos that don't have it at all. Check out
https://zz-2.com/collections/c-play-...uto-interfaces
They are currently listed as "C-Play" interfaces because they are currently an "unofficial" product, i.e. not Licensed by Apple. Get them while you can because if they receive Apple Licensing approval the price will most likely go up.
Dean over at
5 Star Car Stereo YouTube channel recently tested this add-on WIRELESS Apple CarPlay interface with his iPhone in a rental Chrysler Pacifica on an ~11 hour road trip from Clearwater Florida to Tennessee.
He said that it worked flawlessly and he still had plenty of battery remaining on his iPhone after ~11 hours.
Watch the video where Dean describes this...
https://youtu.be/nxIoZv7A10A?t=186