I've gone round this loop several times re. satnavs and dashcams needing 5v via a USB adaptor fed from a switched vehicle 12 volt supply.
'Scotchlok'ing off the radio supply will provide the switched feed; the fiddly bit comes with trying to work out tidy and unobtrusive routeing USB-side wiring for the device/s in question.
Please note that some dashcam product importers/suppliers have associated 'Volkswagen' [Auto Group] with high failure-rate on the plug-in adaptors normally included with the device. No detailed explanation; it just gets flagged up in the Frequently-Asked Questions bit - as here, for example:
https://www.quesh.co.uk/products/?product_id=384
I think the '7805'-based solution is a good one. At least you're more in control of the quality of the end-result
Best wishes
Guy