Rugo,
You actually needed to post this in the radio forum.
If you are referring to the "sponge" seal that goes around the speaker of the radio, you can get those at any of the AD truck parts suppliers. (Jim Carter, Chevy Truck Parts, Bowtie Bits, Etc.) This seal is made of a dense sponge that glues to the radio with weatherstrip adhesive and fills the space between the dash and radio face.
Hope this helps.
Tim
