There's an oem one availble (says it needs coding) but one would assume the wiring would be in place already. I have a sub/amp combo from my old car that I may use. does this mean I don't need to runs wires thru the firewall and to the headunit?
oem one
https://www.vwpartsandservice.ca/p/...-Subwoofer--Soundbox/70516867/000051419B.html
OEM Helix system is plug and play apart from routing the wiring loom from the glove box to the trunk. Only coding required is a dongle to program the correct sound stage as the same part is used across many different vehicles.
Any other amp/sub combo you'll have to run power from the battery to the rear and tap into the speakers with a line output converter.