Custom Job, using switches?

My understanding is completely different.  If I am correct, you want to use both the headunit, and the 4 channel amp to power the speakers.  If this is correct, do not attempt this as you will fry your equipment.  If I am not understanding you correctly, please elaborate.  Explain where the sub fits into all this as well.

You both are correct. Geepherder, how would it fry it. Im using the switch to control the signal to the speakers, either from the amp or HU, NOT BOTH at the same time. That is the easy part, except it cost 12 bucks for the switch.

Austincustoms, yes you are correct, I could one switch or two switches, but lol im pretty sure 3 position and double throw are the same thing. But ya, the question is will it acually work trying to select 1 channel or 1 bridged channel, I emailed my electronics teacher, we see what he says. I need to look at a schematic of a bridged amp and the output of the amp to the input of the speaker, ive searched and cannot find one. If i see a schematic I will understand it a lot better. Man its been about 1 year and a half since ive done analog and amplifier stuff, Im a lil rusty.

Okay, that makes a little more sense now.  3 position and double throw are not necessarily the same thing.  A common Bosch type relay is often referred to as spdt (single pole-double throw).  The double throw means it has 2 positions, normally open, and normally closed.

I've seen some 4pdt relays, but if you use those for switching speaker outputs, you must be carefull your amplifier doesn't exceed the ratings of the contacts.  If you look through the relay section: https://www.the12volt.com/relays/page5.asp#stbm  you will see a couple applications for sub setups: Stereo to Bridged Mono Switched Outputs, and Switching from Series to Parallel and Back.