I will recommend you do not cut any plastic side panels for speaker installation.
These original panels are so rare to find now unmolested and they are so expensive to replace.
If your panels are already cut up then it doesn't matter.
If you have a non fold down rear seat then traditionally you put the speakers through the package tray and you cut the fiber insert and the metal in the trunk. Somebody did that to my car.
That gives you the rear speaker mounts.
In the front you can either install speakers in the doors, or kick panels they make aftermarket molded speaker houses for the kick panels. For the doors you can get factory style grills and make it look like your car came with a factory am/fm radio.
Then you can also install a dual center channel speaker under the dash pad.
This was the factory mount for the am radio mono speaker channel.
My car originally had an am radio so I only got the center speaker in the dash, then somebody later added a sears radio and installed 2 rear speakers on the package tray.
I trashed the entire thing and went back to stock, but I installed a dual voice speaker in place of the mono speaker and installed a oem am/fm radio. I've thought about installed a complete fm system in the doors but haven't found all the pieces, honestly the sound isn't that bad with the stock radio setup, granted it's tinny but I figured it was the way to cause the least amount of damage.
Most of time people cut the rear quarter panels or if they have fold downs they put them at the top of the rear quarter trim or further back on the trunk wall sides.
They have new types of speaker systems you could install under the seats and it's suppose to be just as good as a surround sound system Boise makes something like that.