well I tried several things,
lye
***** n span
foamy coil cleaner ( a product sold in refrigeration supply house. It is an alkali cleaner and strong stuff.)

After each soak I checked to see what came out. Believe it or not, the ***** n span seemed to work best. Each time it would loosen some rust colored junk

This was an aluminum plastic for a small car... not even an
factory part. It was the only one I could find in the yard.

I will try lemon juice next ...to get the soap smell out.

1989 5 door montero 3.0 auto
stock original Japanese
about 200k miles