In the trunk there's that black box that holds the 3rd brake light bulb and the emergency handle if you were locked in the trunk, you know what Im talking about? Well mine seems loose every every every single time I hit the slightest bump it comes undone and rattles in the trunk. I thought this was the exhaust mufflers or something and I was going to go crazy but now I don't know what to do. Yesterday I duct taped the entire thing down and it was good until this morning all the tape came off.

I'll have to post pics to show my point...

 

You may need to remove it, check the retaining clips that hold it in, and replace them as necessary. Duck tape is never a good option on a mustang.

 

The clips need to be resprung and installed correctly.
Mine is a bear to remove and it would never come down from
hitting a big bump.
I cant believe you admitted to using duct tape thats designed
for the HVAC industry on a Mustang.

 

If that doesn't work you could also use a touch of JB weld to keep it in place.

 

do take pics, if im thinking the right thing, i wrapped a couple little pieces of dynamat around the edge of the black box, then clipped it back in. there is no play in the plastic now, it cant rattle on the trunk anymore and it hasnt come loose since.

 

Here's some pics, the tape is temporary of course. The noise drives me nuts...

 

Pull the cover down and look at the retaining clips. You should be able to tweak them enough to have them work properly (i.e without tape!).

 

Yea, what he said....

 

Thanks for the info, when it stops raining Im going to go outside and take it apart and check it out