Does anyone know why there are two wires in an aerial setup (out of HU).
Where do they go? At least one is the aerial itself, does the other just go to ground?

The grounded one is for the shield in a coax cable. It should be grounded at both ends.

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Simon

The adventures of an owner builder in the Tallarook Ranges

I have my stereo under the rear seat so what i have done is got an old aerial
cut the end off stripped the outer of it and then got a decent gauge earth wire
(cause it is black) and solderd it together covered that with tape and shrink wrap
then left the wire a couple of metres long and wraped it up behind the seat to the rear window and tucked it in to the rear window rubber . Works a treat