4 Channel Stereo to Mono Output

[sick472]

First off, I'm nowhere near a stereo guru so be easy on me...I am wanting to make my Sony DSX-GS80 4x100W car stereo drive 4 speakers, but in mono for each. It also has the RCA connections for a sub which I will be using a compact sub woofer under the seat. I assume the sub woofer will not be affected should I succeed in making the 4 speakers mono.

With the fuel tank behind the seat of my 72 C10, I am limited on space for speakers and decided to build a center console to put 6x9's in. One faces the driver's legs and the other faces the passenger's legs. I will be mounting smaller speakers under the seat facing forward. This is not very conducive to the effects of stereo sound hence making it all mono. I am just wanting decent volume and not an unbalanced sound in the cab.

I have read that a single speaker can be made to produce mono sound by using the "+" of one channel and the "-" of another. This is great for those with the single stock speaker in the center of the dash, but can it be used to turn all the speakers to mono?

If I was not so worried about ruining my new stereo equipment, I would try to...

Use the front left "+" and the front right "-" for the smaller left speaker under the seat and the rear "+" and the rear "-" for the 6x9 on the left side of the console. Vice-Versa for the right speakers.

Any thoughts from those with more stereo knowledge?