Spacers are plastic donuts which install between the mounting surface (trooper kick panel) and the speaker mounting flange. (outer ring of speaker) This allows the small-diameter magnet of the speaker to stick into the original 4" mounting hole.

This spaces the speaker 1/2"-1" out from the mounting surface, depending on what spacer is used. I think 3 thicknesses are available - 1/2, 3/4 and 1"

I used 3/4" and it works fine except for 2 things - I had to relocate the hood release on the drivers' side, and had to buy speakers with permanent-mount grilles due to a clearance issue on the passenger side (grille will not install past HVAC blower motor)