0/1 gauge power wire off battery to distro block… split to two 4 gauge wires… one 4 gauge wire goes directly to interior speaker amp… other 4 gauge wire goes to capacitor, capacitor gets grounded. run 4 gauge wire from capacitor to subwoofer amp… then ground subwoofer amp. i ALWAYS used separate grounds for separate amps in my vehicles…might be more time taking and a pain in the ass to do but when done PROPERLY, a shorter, thicker, isolated ground will prove to be better than a shared common ground. ive had PLENTY of systems in my cars… the most radical of which was (4) 12" jl w3’s powered off a 1000/1, a jl A4300 4 channel interior amp powering a set of MA Audio HK-68 speakers off a alpine head unit… i ran all the wiring, installed all the speakers, and tuned the amps myself. and yes… i had a 2 farad MA Audio digital capacitor in there as well…isolated to the sub amp of course.
The way you connect your cap is no different electrically than the way I told him to connect it. The cap is a power source, if you plugged it into the cigarette lighter it would still function the same (if your cig lighter had 4awg wire)