I still got the old 6 1/2 Pioneers (j/k the rubber edge is half dry rotted away) I do have some shallow Blaupunkts that fit in the original mounts without hacking the back off.
I have 6 1/2 all around. Honda stock put in pioneer. I have a DX so no component system, which sucks. If your 6 1/2 dont fit in the front speaker bracket, just take out the black housing, and cut of the back with a saw, its plastic so its really easy. I have seen that the holes in the rear deck are 5 1/4, but my 6 1/2 fit perfectly fine.