first time wiring a new system

Definantly run the the 4ga through the firewall !   Do Not put a D-block in the engine comartment (lot's of problems).  Put an "in-line" fuse no more than 1-2 feet from the battery.  Run the 4ga as far as you can then go to the D-block.  Same with the ground, you want the ground D-block as close to the amps as possible.  Make sure you have a GOOD ground.  don't wrap a wire around a seat bolt.  You shouldn't need a cap.

Also, very important.  Run the RCA cables on the opposite side of the car from the power, remote, and ground wires.

Have fun.

Definantly run the the 4ga through the firewall !   Do Not put a D-block in the engine comartment (lot's of problems).  Put an "in-line" fuse no more than 1-2 feet from the battery.  Run the 4ga as far as you can then go to the D-block.  Same with the ground, you want the ground D-block as close to the amps as possible.  Make sure you have a GOOD ground.  don't wrap a wire around a seat bolt.  You shouldn't need a cap.

Also, very important.  Run the RCA cables on the opposite side of the car from the power, remote, and ground wires.

Have fun.

If I were to skip the distribution block, would it be difficult to add one later on if I wanted to add another amp?  Also, the xtant 403a amp is not internally fused and they require a 60amp fuse installed in line w/the power wire at the battery.  I can handle that, but the manufacturer states that for safety, an outboard 60 amp fuse may be added close to the amp.  Would that outboard amp fuse wire up just like the in-line fuse? and what purpose would it serve if an inline fuse of the same value is already installed?

as far as running the rca(s), can it/they go along side the speaker wires?  I have read numerous postings of running power down one side, speaker wires down the other side, and rca(s) down the middle.  I want to pull up as little carpet as possible as the wife is already nervous about me doing any work on it ('05 Scion xb) as it is only 3weeks old.  Will speaker wires and rcas cause a problem if they are next to each other?  By the way, thanks to everyone for responding/answering my ?'s.  I might actually get this all installed without catching the car or myself on fire.

Input wires such as RCA's or other twisted feed wires can run along with the output speaker wires.  Keep all power wires to the other side, incl. remote turn-on.  Distros and caps are the easiest add-ons to a system.  They each are mounted near the amp and the power wire is already there.  So there is no tearing up the car again.

As the amp is not internally fused, use your fused distro block within 18" of the amplifier if possible. This fuse is designed to protect the amp unlike the fuse at the battery which is designed to protect the vehicle.

Still stand by the no cap necessary at this time. A cap does not make power, it stores and uses it. If your lights are flickering, the first thing to do is upgrade all wiring at the battery / alternator, next upgrade to a better battery and next if it still not good enough is to add a HO alternator.

Rob- Based on what you said, I doubt I'll need a cap or likely anything else for awhile.  I will be using the xtant 403a amp to power some cdt door components and one re-10 sub( sub at 200w rms @2 ohms). The amp/crossovers will be under the driver's seat and the sub box will be under the passenger's seat. Based on how close everything is to each other I should be able to get that f.d.b. within 3-4 inches of the amp. 

Are there any consequences for mounting the amp/f.d.b that close to each other?

 I hope this relatively low wattage (@3-400w) will not affect the lights or anything, but I guess I'll find out what I'm in for when/if I ever add another amp so components/sub can recieve seperate power.