Not sure how to remove the cap from the rod. In the old days, they unscrewed, and aftermarket replacements were readily available at parts stores (That might still be true. Most parts stores today have one aisle with door knobs and window cranks, etc. So you might find suitable replacements there.)On my 2007 Impala the plastic caps on the door locks have basically decomposed in the sun - and are now crumbling to bits.
Two questions; any way to get replacements? (Discontinued parts....), and how to get them off?
You can get the cap, with attached rod, online from parts dealers as well as ebay.
This drawing shows the parts for the front doors:
The rod is item 8 in the pic. The two (left and right) rod part numbers are:
15920295 left, EBONY, TITANIUM
15920296 right, EBONY, TITANIUM
Note there are other part numbers for different colors.
There's no more info - no part number shown - for item 9, but that may be the cap by itself. The rods are on the order of 10 bucks each, so they're not too expensive. That is, you can easily burn 40 bucks of effort pretty quickly trying to find them cheaper
The rear door info is here:
For the cap and rod, the part numbers are the same as for the front door.
Another consideration is going to a salvage yard.