Went to adjust my e brakes (no luck of course) and the drivers side brake backing plate is rusted pretty bad to the point where the rubber grommet/cap that covers the adjuster wheel fell into the rotor when I tried to pull it off and the lower half is in rough shape.

I need to replace the rotors and the back drivers side caliper and my question is...where can I get a new backing plate?
Is this a dealer order item or a local auto parts store item?

I checked Advance website for you...they list one but it doesn't say front / rear so I'm not 100% sure.

I'd take a look there, it was around $25.

I had to put backing plates on my 1500 and it was a dealer only item. Usually, where they are gone, all the e-brake hardware is trash as well

Same issue with my 'ol 99 1500 about 2 years ago, dealer quoted me some stupid number (in one ear/out the other)...needed everything. Kissed ass, barely got a sticker on it, traded it. Had I kept it, I would search aftermarket until my eyes bled...common enough problem you would think someone else makes them by now, no?