I tapped the wire under the red cap, but on the outside of the housing. The white wire exits on the driver's side and runs to the 12v battery under the hood. I cut it a few inches up and used a distribution block to connect my 12+ input to my 900-watt class A/B amp and it's working very well so far, driving 2x woofers, 2x tweeters, and a 10" sub. The ground point you see under my red wire is the PERFECT ground since this is the DC-DC converter's output from just about the red rubber cap. I added my ground here after this pic was snapped.
Here's what the wire looks like with some of the loom stripped back:
I was toying with the idea of running my amp power wire to under the rubber cap- hence why you see the red wire running into the housing, but I decided tapping it on the outside was the better way to go since the distance was so short.
This is how it looks at the moment:
I have not permanently mounted the distribution block yet since I plan on replacing that red-jacketed copper-clad aluminum wire that's marketed to car audio installers with 4 gauge Radaflex copper welding wire from cableyard.com: RADAFLEX® #4 AWG Black Welding Cable
I found it silly to run a power wire from the 12v battery under the hood all the way back to my amp as is done in a traditional installation. The 12v power source for the Insight is in the back (the DC-DC converter) and my amp is going to be installed right behind it.
Running a capacitor is a debated subject in car audio forums. This one has the most meaningful information I have found so far:
Capacitor Myth - DIYMA Car Audio Forum
Most will tell you they are a waste of time, especially since most modern amps have a bank of capacitors integrated into the design already to steady the input voltage during moments of peak current draw. I have run capacitors in previous installs, but the money would have been better spend on a battery instead. I will be installing a 2nd 12v battery in the rear of my Insight, but it will be an AGM type and I'll exchange the front 12v battery for the same. Mine's an '03 model and the old-school lead-acid unit is probably due for the recycle bin anyways.