I have the AVH-4000 nex
It is OK, But it is unreliable, lots of re-starts and super picky about lightening cables.
Mine may be flaky, I've only had one unit.
Have two units in the household, no issues with either one. The 4000NEX is a very good unit. Also, it has been superseded by the 4100NEX (which shouldn't have any of the flaky issues you've found).
The 8100NEX is where it's at for a nice capacitive screen and navi, though with CarPlay you don't need an internal navigation if you connect your phone all the time.
Any way you go about it, use Enfig parts for the adapters, and the MaestroRR for the car integration.
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Edit: After seeing your other thread, it seems you could use a bit more help... Here would be my suggestions:
Antenna Adapter:
HERE
GPS Adapter:
HERE
Dash Kit / Adapter:
HERE
Microphone Replacement:
HERE
Enfig's Install Videos:
HERE
iDatalink Maestro RR:
HERE
iDatalink VW Harness for RR:
HERE
Pioneer 4100NEX:
HERE
or
Pioneer 8100NEX:
HERE
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If you don't think you're up for the challenge of the install yet, watch some videos on installing and see if you can't get a bit more comfortable. If you need specific help, post back or PM me.