Here's the original plastic VDO washer reservoir from a 1963 Mercedes 230SL. 50 years of dirt and grease shows.
I used the Comet with Bleach no scratch formula. You want to clean the part but not to scratch it much.
I did this in our kitchen sink. I got the part wet and sprinkled the cleaner on it. Since there's bleach in it you can let it sit for 5-10 minutes before you start. Make sure the cleaner is wet.
After the first cleaning it's looking better already. Repeat each side a couple times. Don't scrub too hard or get too aggressive with the scotch scrubbing pad.
Finally, use a few teaspoons of vinegar and some water and let it soak on the inside. It will clean out the old scale that's built up.