How to clean window regulator grease?

Find a tub that's big enough to immerse the entire unit.

Mix #2 diesel fuel 50-50 with the cheapest ATF you can find.

Soak the regulator in the mixture making sure the entire unit is covered....a day or two should be enough.

You'll be amazed at how well this works. I learned this method when I was a kid working in a farm repair shop. You can imagine what kind of caked-on greasy stuff people would bring us to repair from machinery and trucks and stuff....