When I stripped my 61 I found there was a pair of rubber tubes that went from the corners of the parcel shelf, into the very back trunk space, and draining out forward of the axle.
These were very brittle and were falling apart. The formed top fittings just fell off.
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I have never seen this from a Cadillac seller, but I was looking around at a Chevy seller Cars1nc and they list a few. They are cheap enough I will buy all of them to find a set that fits.
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Years ago, when I restored my '56 de Ville, I bought drain tubes at a Chevrolet store.
I did too (for my '60) from a '57 Chevy, which have everything available online.