Welcome. Um let me ask, is your aftermarket head unit (hu) a 1 din chassis? if so, it'll fit perefectly into the slot. just take off the orig dash piece (carefully, not to loose any clips) and take out the factory deck, and use the existing mounts (screwed into the side of the factory deck) and they'll screw into your pioneer hu directly ( or they should atleast ('it's an industry standard to have the screw holes drilled at the same places on all decks that way you can use existing factory mounting set ups for aftermarket decks)
I just did that, bought a wire harness for the new deck, and connected easily to the factory wiring assembly.
you may have to tape the cd holder up to the underside of the new hu, so it stay in place in your dash.
If that is confussing, please let me know.
Clockwork