ports in bulk head

on my 110 commercial there are two holes in the bulk head behind the front seats these are filled with panels riveted in im not shure what they would be used for but here is my cunning plan i have a pair of vibe qb69 speakers taken from my old car they give great balance sound without needing a subwoofer if i take out the blanking plate i think they will fit in the resulting holes i can then fit some form of panel to the bed side of the bulk head running straight down to the floor to end up with a speaker box is this likely to cause me any problems and just out of interest what are they actually for

Might get trouble with vibration, that metals thin as hell and will probably rattle alot with a speaker there making a terrible noise! Don't really know what thier for tbh!