The pilothouse doors in my 1990 MY have had the same problem. The door rollers on the bottom track have a plastic wheel with a non-stainless steel bearing that rusts. I've replaced a few of them over the years, which requires taking the door off by unscrewing the bottom door track from the boat. There are no bearings at the top, only the two at the bottom. There are also two smaller rollers on the bottom that are in the kit sold at Sam's. Sam's has the kits in stock, but they aren't cheap, about $65 per door as I remember.
Load them with LPS-3 or some other corrosion protective lubricant to extend the time before the new bearings rust too! I think that if the bearings were lubed every year they would last indefinitely but removing the doors annually to do that is too much aggravation.
It would be great to find a stainless replacement bearing, but the plastic wheel on the outside of the bearing is required too.