This is my experience from today - in case anyone else has a similar situation...
On my car, the front turn signal light lenses are still basically OK, but have turned yellow over the years. I thought that the plastic itself had yellowed, but after cleaning it, it turned out that the color was from dirt deposited on the inside of the lenses - mostly way down in the corners and around the seams that you can't reach even with a long skinny brush.
Problem -- they are glued together and I didn't want to break them open. I tried filling the inside and shaking with soapy water, various solvents, lime-away, etc.... you name it. They all helped a little, but not much.
So, as a last try, I filled up the cavity 1/4 full with sand, then added some liquid laundry detergent and water. Then I shook it back and forth for about 5 minutes and all the yellow deposits were removed. (You can't shake it fast or the sand won't move all the way -- shake slowly and let the sand move all the way to each end.)
I was surprised that this didn't scratch the inside surface at all. But if you try this yourself, be sure to add the detergent. If you just use sand and water, scratching the surface is more likely.
Well, that's it. Not that big a deal, but it saved me from having to buy $100+ of plastic lenses.
Fred