Great questions.
I would think grabbing another Fosi would be the way to go. As for installations behind the dash, not a simpler ir easier task; on the contrary. Consider running the large lead to the distro block under one of the seats.
Then run the wires over to the other side, under the carpet, to the two amps, of which ever you end up with. Now, if you do elect to mount them elsewhere, even up under the dash where you have access without removing much of anything, then consider using this amp for the front midbass and the rear coaxials.
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That would free up the Fosi to deliver 100x4 to the tweeters and the rear 4x10s. You would just set the gains conservatively for the output to the tweeters. Additinally, since that channel will not be split, you can also attenuate it up or down after the fact to level control it's specific output relative to the volume of the midbass.
The cap placement for the tweeter protection is as follows:
Somewhere in that lead, closest to the tweeter you can manage.
I did want to mention something that is important to be aware of and write down as you get started.
Always look at the crossover grouping on the amplifiers; these are always paired. You will see them listed as front & rear L/R or 1/2 & 3/4, etc.
Pay close attention to how I routed the split signals to the various amplifier channels.
When you run a signal to the amp, write down what you're doing. I ran the front head unit signal to the Fosi 1/2 channels; left is 1, right is 2, etc. Do the same each time you get ready to plug things in.
You'll do this so that when it comes time to wire the speakers, the only important thing, is the L/R part and which driver it is going to.
Having marked what is going into the amp and for what, ensures that you know which speaker lead is going where as well.
You just need to make sure that the signal going in gets to the speaker it's intended for, and that will go a long way to making sure it happens the way intended.
I bring this up cause even though I've advised on what to do with the HU signals coming from the RCAs, the drawing, relative to the actual connections to the amps, may be reversed or whatever, but as long as you write it down and make sure that the speaker leads associated with that input are used for the correct speaker, you should be fine.
As for the wire gauge, the important thing to remember is that the distance from the actual power source and the current needed to run that amplifier unimpeded is what determines that actual gauge needed.
If you can get the combined current needed for all the amplifiers to the distribution block using 4 gauge, great; if not, you need to move to 2 gauge; if not, move that length of cable to 1/0 gauge, etc.
Now that you've got that established, then most of the amps we're talking about, and the sub (which is a powered sub), will likely be fine with 8-gauge power/ground leads, as long as they are close and as long as they can get the power needed, uninpeded/reduced from the distro block.
There are many charts online that will tell you what a given length cable is capable of easily carrying the current needed at a given voltage (use 14.4).
If you're right there on the total needed, always opt for the next largest unless you cannot physically get it there; if really close, you're going to be fine considering that you'll likely never be playing at anywhere near system potential theoretical max draw.
As for the fusing, yes, always good to match fusing to the amp as already stipulated by the manufacturer.
That said, it's not necessary if the amp already has built-in fuses. Then you can use whatever came with the block or a dummy fuse (neutral link); basically, it is the shape of a fuse but just a piece of metal instead of a fuse that bridges the gap.
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If all the amps in use have their own fusing, then you really only need a 1 to 3 post distro block and a 1 to 3-grounding block
Power and ground:
- OR -
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Don't ever strip down the wire if it's too large, not a good idea, use reducers.
Brass or plated copper is best here, as aluminum will oxidize and corrode where brass and tin-plated copper will not- well, not nearly as fast. Tinned, solid copper is best, but it's also hella expensive and rarely used.
