"Superstition" by Stevie Wonder is indeed an awesome, classic pop tune. :)

The sound you are hearing in the main riff does have some clean guitar in it. But what gives that lick it's signature timbre is Stevie's use of the clavinet.

http://en.wikipedia.org/wiki/Superstition_(song)
http://en.wikipedia.org/wiki/Clavinet

I've played that tune at least 1,000 times. Lots of fun. :) I usually just use a clean Fender guitar and amp type tone with the out-of-phase position combining the bridge and middle pickups. Kick in the wah and find a place to leave it set; don't rock it back and forth. Although, I have done that too and it sounds pretty neat for a variation on one of the verses.

That should get you a close approximation. Hope this helps.