What is it?

There is a small box mounted to the underside of my rear deck in my 98 cvpi. What the heck is it?

If deck you mean trunk lid. It is your fuel pump shut off switch. What it does is in an accident it shuts the fuel pump off which stops fuel from going to the engine so a fire does not start.

I found out what it was, its the antenna box. Its mounted next to the right rear speaker. Thanks everyone.