Eh what the heck, couldn't find an answer anywhere... but found out now that the speakers I have are too big. But I installed them anyways... it's precisely that they fit without seeing they are too big (seiling is pushed out 1mm). They didn't fit the slots at all... but I used screws at a 45 degree angle. Should stay in. :-)
If you're wondering, I installed 6.5 inch speakers in the front and rear. Stock size are around 5 1/4th, I guess.
Oddly enough, these new "pimp" speakers sound clearer and you hear MORE of the music and beats, but have hardly any base to them at all. Fronts are 2 way with base and treble, the rear is base only with optional treble (tiny extra speakers).
Probably gonna have to install a subwoofer.