Possible SOLUTION FOUND! Bought a 2021 Ram Rebel about a month ago. Dash rattle started and was driving me bonkers, I spent a long time taking things apart trying to find the source. The noise sounded like a retainer clip, springy kind of noise every time I hit a bump, eventually though it became almost constant. The noise seemed to be coming from behind the driver side vent, but after much searching I found the noise repeated by hitting the dash beside the electric park brake switch.
Voila.....followed the noise and thought it was the wiring harness connector.....however it appears that the e-brake cable itself has a very annoying rattle to it. Ram made an attempt to insulate the cable and secure it, however this still allowed much movement to the cable. My home made solution was to wrap the cable with two micro fibre clothes around the cable and secure it from moving. I'll be taking it to the dealer to find a more suitable permanent solution, however this may solve several people's problems with what seemed to be a dash cluster rattle or vent rattle as the noise seems to float up the dash.
Hope this helps! I'm now rattle free!