First, the more I read about car audio, the less I understand. Something about it just doesn't click for me, so I apologize in advance for asking it be broken down like Im 9.

I will be buying a Custom Auto Sound stereo for my truck. There are no other options I want, so please dont suggest a different head unit which might fix this issue much more easily.

The CAS has 4 channels, FL, FR, RL, and RR. The speaker set-up I am trying to achieve is a 3 step process -

1 - Left door, right door, and the 6x9 center dash in Mono (Front Channels)

2 - Above, then add a small sub (no or limited fade impact) This doesnt need to be anything more than a single 10" to fill out the sound a bit. The cab is small, and rattles like crazy already.

3 - Above, then add in the 4 speakers in a Highliner Headliner. (2 on the LR, and 2 on the RR channels).

Given what I have, how do I get the front center to operate in Mono?
How do I keep the Sub from getting faded front to back?
Could this be as easy as buying a 6 channel amp out of the gate and setting it up to run the 4 corners plus a Mono front center, and sub?

 

The center channel is going to be a pain no matter what. Modern stereos aren't designed to do that, and trying to isolate the signals will not work without some kind of processor.
So start with the simple, use the rear channels as signal to your subs. If you're just going to run them/it in mono, then just one rear. This will allow you to use fade to adjust the "loudness" of the subs and leave the amp gain adjusted as it should be.

Unless you are working with high ohm speakers in parallel or normal speakers in series (which will cut your volume sharply), you will need to power the extra speakers with amp. The 6 channel amp will give you the channels you want, but you will have to pay attention, the 5/6 channels may be low pass only and not what you want for a center channel. Not to mention having a center channel powered by either the right or left channel will blow the stereo imagining out of whack.

However in total you are talking about 8 channels total. You are going to need pre-amp out puts and I would look at two 4channel amps. I'd also get a crossover that would support 7-8 full range speakers plus the sub channel.

I know you've said you're sold on your current layout, but you can achieve excellent sound with fewer parts and less headache. Unless you find a crossover that offers center channel, you'll also have better stereo imaging with fewer speakers in better locations.

 

Its not so much the configuration that Im sold on as the radio itself. Its really the only one that I have been able to find that keeps the old style radio that better suits my truck. As for the speakers, I have one in the dash, one in each door, and plan to get the highliner (which comes with 4 speakers) for other reasons. Figured that since I have them all, might as well use them, but now its clear why I couldnt figure out how to do it on my own.

 

I've have only summed outputs on one radio, it was an older rockford fosgate. The outputs were RCA LF,RF,LR,RR and a pair of SUM. The summed output is what you need for the center channel. You can probably find a cross over or processor that can give you that. Subs are typically summed, but the problem with using a sum out from a crossover is it will either block the freqs the sub shouldn't get or you will have to all pass the channel and tune out the highs at the amp.

Truthfully, the locations of speakers will cause the sound to get muddy. The left headliner speaker's sound will reach your ears faster than the left door, next closest will most likely be the headliner right and then door right. You will probably get a good deal of destructive interference which will make the system sound quieter than it really is. If you get a higher end cross over or processor, you may be able to tune the phases to help a little.

That is all assuming you're into high fidelity audio. If you're just looking for the CDI factor its not that important.