I have some old pics I took for someone a while back...
Stock in-dash tweet:
The in-dash tweeters are 4" from mounting hole to mounting hole. You're going to have a hard time finding anything decent to fit in there, though...
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Stock front-door 6x9:
You can definitely replace these, you just need to take the whole mounting ring off and then remove the speaker.
To get the door panel off, remove the screw cap near the front of the door (in the indented portion where it meets the dash) and unscrew. Pull the rubber bottom out of the pocket in the door, remove that screw too. Pull out/disconnect the wires from this plastic pocket piece. Then carefully pop the piece of plastic inside the door handle (pull the door handle/lever out, the "backing" piece behind that) out and remove that screw. A few plastic clips, then lift the door off, and you're good.
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I just realized I don't have any pics of the rear speakers. To get at them, you'll need to fold down the back seats, and if I remember correctly, remove the side seat supports (padding left on the sides after the seats are folded down). They're held in by a bolt at the bottom, then give them a nice push up to remove. Then you'll want to unclip the 3rd brake light and unplug it. Two more white plastic snap-in clips under the back deck (above the trunklid's torsion bars), then pull the back deck cover out up/toward the front of the car to remove... and there are your stock rear speakers.
You'll be really limited to what you can do by only utilizing the stock speaker config in the Gen5... if you want anything that sounds outstanding and is staged properly, you'll definitely need to go custom.
I have some pics somewhere (can't find them now) of my current setup... 4" midrange and 1" tweeters in the dash, 8" midbass in sealed fibeglass enclosures in the door. With the door cover on and the grilles on the dash, it looks bone stock. Let me tell you from experience, I found it easier to custom fabricate mounts for the dash speakers than to try to make things work just using what would fit.
If you have any questions, hit me up in here or PM me...