WARWAG,
Bought a M1028A2 last summer. IMMEDIATELY took the cap off the rear once I got it home.
BUT, I kept it. I'm glad I did. I found that during the winter, the only thing the bed does is collect snow. So with the cap back on, it will remain dry. At least that's the hope.
So yeah, pull the cap. But don't pitch it just yet. You might find a use for it.
I did.