Love it! That's exactly what I am doing and what I have going on. I have the 4 gauge running to the trunk. I'll add a power block then tap off of that. So 10 gauge wire from the power block will be sufficient for a moderate powered sub? I have extra 4 gauge, can I run 4 gauge off the power block to the main amp? Too large, does it need to be smaller off a power block? I've done a lot of wiring but never messed with a power block, simple yes... but want to make sure on sizing.
Thanks guys, super helpful.
In my years of doing car audio, I limit 10 gauge to a maximum of 30A, 8 gauge to 60A, 4 gauge to 120A-150A, and 1/0 to about 250A-300A.
In your case, the total draw with the two 30A fuses and the single 15A fuse adds up to 75A. 4 gauge copper wire can handle that draw and while a 75A fuse might be readily available, a 100A fuse is probably more easily found and that's why I put that on the diagram. If later you change out something in back and the total draw is 120A, then the 100A fuse could be changed out to that size and still be safe on that gauge of wire.
On this setup, if the 10 gauge wire were to short to ground, the wire not being large enough to safely carry 100A could suffer from melted insulation before the underhood 100A fuse would blow BUT the 4 gauge should show no ill effects as it should pass 100A all day long and be fine. It can be more technical than this but just remember size the wire too the draw and there are plenty of charts out there online. If you have 15A draw, then 10 gauge or larger is fine BUT never run a 100A draw off of a 18 gauge wire or smaller.
I always like to fuse as close as I can to the source of power and with ampifier's containing built in fuses you can do redunant fusing but added connections can create issue. I have before taken an original 20A fuse out of an amplifier and replaced it with a 30A BUT I then place a new 20A before that 30A. This way things are still protected but in an installation that accessibility to an amplifier's built in fuse is hard to get at, this works fine. You just have to think things through.
Just to throw things out, and who knows what the combo you have will sound like, is normally I like to have a 2-1 ratio for amp power between the sub and the full range speakers. In other words if I have 400 watts total output on a 4 channel amp to speakers, then I like to have at least 800 watts going to the subs. The problem here is on your system it may be fine due to speaker effeciency and loads (2 ohm VS 4 ohm VS ??? ohm) that this ratio is out the window.