I'm sure this might sound dumb to most.

But I've decided on my spl/show off aspect of my stereo to be only outside, when I want it.

Inside I have my decent entry level SQ setup with time alignment.

But for those times when I want to turn it up and share with the world, I want to have a 6.5" pro audio driver (250w rms, 150Hz-8kHz) along with a soft dome tweeter (50-80w rms, 4kHz-20kHz) just behind the rear windows so I can flip a switch to turn the secondary 4 channel amp on, and simply roll down the windows.

Like I said, I know this sounds dumb. But the alternative is loud speakers inside, which would be much less sound quality for me and also ruin my hearing (even if only for a few minutes - my hearing seems to bounce back shortly after)

I've inspected the inside panels in the rear and so far the seat belt bolts seem like the best option. There is also a decent lip inside the panel I can use.

I don't think the door itself would work as the only visible part on the outside is the window rubber. No metal exposed when doors are closed.

I pretty much just need to get a flat piece of plywood/mdf mounted either same slant as the window or level/plumb so they shoot straight out.

 

My answer will also sound stupid probably, but if I understand correctly:
1. you need speakers that broadcast your sound to the outside world;
2. You don't want these speakers spewing sound into the cabin, you'll be deafened by it.

Logic tells.Install your speakers in such a way that they spurt sound outward and do not reflect it inward (take care of your ears). Install them under the hood, or behind the bumper, or behind the grille, or find another suitable location. Lots of ways to be creative. Experiment with sound-heat-insulation and other modern solutions.

If you consider audio systems for marine boats - it will be more expensive, but more powerful and protected from getting wet.
If you throw out the foglights and stick loudspeakers in their place, you will be known as a reformer with out-of-the-box thinking.
Well, all in the same vein.

By the way. Why do you want to share your sound with the world? Ready to even spend big money? Just wondering.

 

The speakers will only be activated when I turn a switch on, which will turn the secondary 4 channel amplifier on to power them.

I thought about outside the vechicle. They always sound like garbage and nobody would be dumb enough to put expensive speakers out there.

As far as expenses go, I've always got the mids and tweeters, just need a decent 4 channel amp. ($100-$200)

if you've never rolled the windows down and turned the music up as loud as possible because you wanted other people to hear it, you wouldn't understand

 

Sometimes I see cars with enough powerful speakers and a subwoofer, incl. those that direct the sound outward. I understand the desire of some owners to provide the world around them with their musical preferences. Sometimes I like what I hear. Sometimes it's not very good.
In any case, the decision to equip the car with a non-standard and powerful audio system requires a very professional approach, it requires taking into account the slightest nuances. It follows from your words that you also understand this. Perhaps this is not the first time you have installed music in a car and have relevant experience.
It seems to me that your suggestion for installing wood panels in windows is generally correct. But you should pay attention to the rigidity of the fastening, preferably directly to the metal. In addition, one should take into account the possible vibration of the sheet metal of the door, as well as the door itself relative to the body - the door lock has some play. You will have to inspect the door seals. It will be necessary to remove the door trim and somehow press the sheet metal, while ventilation should be taken into account to avoid corrosion. In addition, the door skin itself can vibrate relative to the door itself. In general, all this should be prevented. After all, you need a clear sound without the accompaniment of rattling goblins.
I wish you good luck, I am sure that everything will turn out well for you in the end.

 

Use good marine-grade plywood. For pretty I like the teak/holly for cabin soles.

 

 

If you ever lived next door to someone that did, you wouldn't understand.

 

Its gonna be covered in dark felt and then im gonna make a black grill cloth cover so you can't even see the speakers or the wood. It I'll just be dark. 😏

 

OK. We are waiting for the video, it is interesting to see the result