Looks like a solid enough cable. I would still prefer the go from the frunk for a direct battery connection. Who knows what else is running on that wire before it gets there. I believe I've seen a way to get through the firewall under the drivers side. Its a easy run to get a amp back there. Now you might run into some problems since the audi amp doesn't use ground wires like a normal car. So I would think you'll end up running wires anyways. Audison makes amplifiers that work with the audi head unit from what I understand.
I really hope you have not done this, I was told in no uncertain terms that this is a disaster waiting to happen, that cable is only rated for the power draw it is likely to encounter. I don not thing th factory amp is powered off that and it is not a powerful amp anyway.
A power cable for a decent amp would be getting on for that size and that's without it running things like the starter motor which are high draw and the engine ECUs are back there too, DO NOT DO IT is my advice
It is easy to run a correct power cable from the battery, I did this it's a couple hours work as follows:
Steps 1-2 there are loads of detailed guides on here, search for 'change battery'
1 - Remove the wiper arms with a puller tool
2 - remove the 4 plastic pieces of trim in the front trunk and then the tub disconnecting the light power cable
3 - there are holes through the bulkhead that you can pass the power cable through. I used one that comes out behind the glovebox. It had a grommet in, just make a hole in the grommit, poke the wire through on the trunk side and feed it in. After you've fed a load in it will start to accumulate behind the fuse box - you can see it from the passenger footwell if you shine a torch up. You can remove the 6 screws that hold the plastic cover but it's not necessary
4 - take the fuse box cover off (4 screws) and take your wire out, pull it taught
5 - Pop off the plastic door sill cover - just pulls up. The plastic rivets can break - buy a variety box of 200 for $5 from eBay so you can replace any. There is loads of space to pass the power cable under there & from there up to the bench area where you can remove the factory amp & install your own
Don't use the Audi amp's ground cable it won't be rated high enough! Just ground to one of the many metal chassis points under the rear shelf
Thanks Jake I appreciate the info