Copying my original post from "What have you done to your DeLorean today?" to here so easier to find for others in the future.
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I replaced the disintegrated sponge rings by the rear air intake/vents.
For less than $8.00 I picked up 2 yellow Mini Sink Plungers from HomeDepot (also available at Lowes). Removed the rubber rings. Cut & placed around the intake tubes. Seam placed in back corner and is not visible. I thought of adding some black RTV to secure in place but haven't yet as they don't move and stay in place just fine.
All credit to Chris Toller for this find.
MiniSinkPlungerRing-1.jpg MiniSinkPlungerRing-2.jpg MiniSinkPlungerRing-3.jpg