I have a 1999 plymouth breeze and just installed 2 12s and there is a really bad rattling coming from some sort of plate under the trunk, not the trunk lid or any of that, but the actual plate under the trunk is rattling really bad. is there any way to stop this? or completely remove it without hurting the car?

 

The plate is probably the heat shield. You can try a couple different things.
If you can drop it you can try to put a small rubber washer on each of the bolts to give some seperation between the metal. If the plate is hitting in the middle and rattling there, you can try some dynamat where it's hitting, to deaden the sound and stop the metal on metal contact.

Also make sure there isn't anything loose in the spare wheel well, as that can make it sound like something "under" the car is rattling. An old blanket, bath towel, or rag can help keep the spare tire and jack set from rattling too.

Rubber grommets and washers go a long way towards a "clean" bass hit