What is this wire.....
 

........and should it attach to anything? I took off the bumper last night to replace the exhaust and this small wire was running through a hole in the back of the body with a loop on the end. It was not attached to anything, just laying on top of the heat shield.

It looks like it could be the emergency boot release cable.

Just in case anyone finds themselves trapped in the boot (dont see how) they should be able to pull the cable to release the catch.

Have you tried pulling it?

It is the rear trunk emergency manual release - similar to one for the front trunk. In case the remote or switch (or lever depending on MY) doesn't work, you can reach in thru the driver's side rear wheel well liner and pull it to manually open the trunk.

Not to be nosy, but that looks like a PSE exahust. Are you replacing a PSE with something new or is that the new exhaust?

The PSE is the new exhaust.