Honestly, its hard to properly install oversized speakers in the rear corners in my experience.. You can definitely fit 4's all day long, i have 5.25's in mine. Try turning the speaker, the little tab where the bottom screw goes in for the stock bracket is usually what keeps you from squeezing a bigger speaker in. if you can get 1 bolt in properly, tighten it, and even ziptie two more it will usually hold pretty securely. That's what i did and the speaker hasnt moved at all.

 

Did you grind out the speaker hole to fit the wide body of the 5.25 into the rear speaker hole?
My 4" Pioneers are a tight fit, and I was hoping there'd be a bracket adapter available or some kind of speaker designed to bolt straight up to the existing factory location. I'm guessing that there may be no other choice but to get it happening by using one bolt and some ziptie improvisation like you said to get it installed

anyone else with their stories/advice/experience at fitting rears in their sw20?

I just want to replace the rears with some cheap aftemarket speakers that is easy and simple for the car stereo novice

 

For my 5.25's, i bent the tab back slightly (not necessarily for fitment, mainly because i put a $40 speaker in each side and don't wish to risk damaging them, lol). But when i had 4" speakers in the rears, there was no issue. I simply turned the speaker until it pops in (you know how there are 4 pieces of metal going from the front of the speaker to the back and there are openings right behind the cone? line one of those openings up with the little tab that is keeping it from fitting. It doesn't hit the cone or anything, don't worry) and like i said, a screw and a few zipties. Ive mounted a few speakers that way and done a lot of aggressive/crazy driving and theyve never moved a bit.