I have Jetta 2004 with a brahma 12"mk11 in the trunk. How do I stop the rear deck from rattling
or vibrating

 

it mybe the seatbelts
sound deadening like b-quiet ,brown bread

 

still on topic. what if it's the plastic cover for the brake light that rattle's, how would ya stop it from doing that

 

With any inside panel the key to stop rattles is to get the peice to stop vibrating. The obvious solution is to place a material between the panel and the car itself. If the panel can't move it won't vibrate. Sometimes this means taking off the panel and attaching something to the underside - I use carpet underlay. Other times it involves tightening the panel down (if possible).

If you can remove the panel, do so and hear if the problem goes away...if so then you need to get creative - find something, anything to wedge between it and the part it is rattleing against.