I'm still in the learning phase of this but I've got a couple of things figured out.
1) Where to position the mic
2) Good audio makes videos 1000000x better
I helped
Montaver set his mic up on his GT4 at NJMP last weekend and the difference is incredible.
This is the mic I recommend to everyone, at least initially:
https://www.amazon.com/Purple-Panda-.../dp/B01MSKI46R
Excellent price, comes with every adapter, extension cable and a dead cat.
Above is the best location that I have found and it has been pretty universal so far. I route the wire through the trunk, close it on top of the wire and tape it to the top of the license plate just below the rear camera ledge.
Above is a clip of my Z06 with the purple panda, through a GoPro Hero 9 + external audio mod. GoPro Mic set to STANDARD MIC.
This clip is my R8 with a Rode Wireless Go mic, through a GoPro Hero 9 + external audio mod. GoPro Mic set to STANDARD MIC. Receiver set to minimum input dB. Same location on the rear license plate.
The Rode Wireless Go is a pricier option, I don't notice any real difference in quality but it makes it more convenient if you don't want to run wires through the car... or in my case - I can't route a wire through the engine bay.
https://www.amazon.com/Rode-Wireless..._dp&th=1&psc=1
If you've got tips/vids/audio - please share!