I tried a few odd ball extra electrical end plugs in an ultrasonic cleaner several years ago as a test. The original white connections were cruddy brown and stained. They came out beautiful and white. However, they were cut off ends and just testing the effectiveness of ultra-sonic on rubber pieces. I would never put a wiring harness in a tub of liquid. See Midlife's comments. What I learned is it works great for small plastic, rubber and fiberglass trim, grommet pieces and metal bolts etc... I would consider using it on plug ends only if I could guarantee I could seal off the inside of the plugs, not sure that is even possible. It works great on small nasty rusted suspension parts if you use citric acid for degreaser and de-rusting.
Otherwise, I love my sonic cleaner for all kinds of little do dads!
They come in all kinds of sizes.
Just my experience and two cents.
Dan in Kansas