This may or may not apply to the Pioneers, but I'll throw my info out there.
In my 93 the stock speakers screwed into the factory "housing" via a screw on the backside of the bracket going into the backside of the speaker magnet. The speakers I bought (4" Boss blah blah blah top end for them) had a simple 4 point screw flange 'slots' that many speakers have....
I ended up using a 3.5 (maybe 4"?) hole saw in my drill press and drilled the backside of the stock speaker plastic housing.... so basically I had the stock mounting flanges but nothing on the backside of it.
This didn't resolve the flange attachment issue - so I went a tad unconventional.... I used (had lying around) Copper high temp RTV (dries solid) to "glue" the speakers to the stock flanges.... worked flawlessly, and if anything is now acting as a damper - no rattling.
The speakers I have didn't fit with the stock grills 100%, but about 95% (they are there, but a little cockeyed).
Unfortunately I don't have photos because it was one of those "kinda fed up with the options I don't have... and now to try something weird.... and don't really want to document it"....