is it still installed or have you removed it?
if it's off the car you can do one of a couple things. Transmissions shops will usually clean them for you for under $20 in their case washer.
the last one I cleaned I just used a couple cans of carb cleaner (99 cent at autozone) and sprayed it real good. Then I took it to the nearest car wash, set it on a mil crate and blastd the heck out of it. Did a good job too.
Another option is bead blasting.
If it's on the car you'll have a tougher time with all the stuff in the way.
