Alright, so here's what I've got so far.
I've added my coreystestcrate.dff to the gta3.img file, along with the TXD just fine. That part I can handle.
What I keep getting confused at is where does my collision file go? I've tried merging the coreystestbox collision with SFw9.col (where my crate should appear), and just dumping coreystestbox.col into gta3.img (which crashes the game until I take that file back out).
My question is, where on earth do I put a COL file after I've made the model? I can't find any help with this online.