I used this exact spot for the pos.
And for the negative, I attached it to the battery - right under the seat.
I don't see where I went wrong. Does wire size matter?
As a "recovering" audiophile, there are a couple of issues you need to address.
First, your power (Positive / Red) should ALWAYS come direct from the battery (not the block where you have it connected). Use a Ring Terminal (auto parts store - appropriate size for wire gauge) and connect it directly to the battery post. Yes, the Touareg battery is in a terrible location for this....
Second, your ground should NEVER go back to the battery - always to the chassis (metal on the car). Make sure that you scrape the paint off wherever you put your ground in. AND make sure you look underneath it so you don't put a screw into wires, a gas line, gas tank, etc. Your ground should be short - like 2-3 feet. Long enough to get to the chassis only. Again use a ring terminal and run a self tapping screw into the chassis close to the amp.
Lastly, wire size absolutely matters. The required wire size (gauge) is based on two factors - amps required and length of run. Use a wire that isn't big enough and the wire will get hot (hot enough to melt down sometimes). I get it's not a competition system. But I wouldn't run an amplifier in a car with anything less than 8 gauge wire.
I've had several systems over the years. My favorite system had one run of Ought Gauge (0 gauge) wire from the battery to a big capacitor and then on to distribution block to the amps. It pounded.
My favorite installation (I sold 12V stuff for a while) was an early 60's Ford pickup that we did. Filled the bed up with 6 18" DVC subs and their enclosures, 6 giant mono amps and then cut a custom vent the width of the bed from the bed into the pickup cabin. It made your eyeballs giggle. It hit 152db. Crazy stuff, but that's what he wanted.
Back on point, here's a link to a handy chart on wire size. I googled it.
http://www.sunforceproducts.com/Sup...harge Controllers/WireGaugeSelectionTable.pdf
If you get your wiring wrong (generally with the ground), the biggest thing to look for is a ground loop. What you will hear is static, a buzz, or sometimes even a whining sound that goes up and down with the engine speed. The easiest way to solve that issue is to move your ground point or make sure it's actually touching bare metal and not painted metal.
Good luck! You'll do fine and really enjoy it!