This effect used to puzzle me a great deal when I heard it as a child, because I couldn't imagine how it was made.
In addition to what's already been said: I don't think an autowah can be used to create that specific wakka-wakka scratching effect. I've tried it, it sounds indeed vaguely similar but not the real funky thing that blows your socks off.
I think that's because an autowah is triggered by the envelope voltage: it will be on the "AH" side of the "OO-AH" vowel scale when the output of the guitar is strongest, which is more typically when you play an open chord, rather than quickly strumming (wholly or partially) dampened strings.
It's exactly the noise component added by dampening the strings that sound most like voice phonemes when filtered because of their noisier / richer range of frequencies. This not only gives you the "OO" and the "AAH" phonemes but a whole range of plosive sounds like "K", "B" etcetera - hence the label "wakka guitar" that's sometimes used. When I first heard it I thought of it as "that kookabakka sound" :)
I've also experimented with vocoders, and there it is similarly important to have a signal that is rich enough in noise and / or harmonics in order for the vocoder filter to have something to sink its teeth into, so to speak. For instance, a pure sine tone cannot be filtered to sound like a vowel at all. It will only vary in volume when used with a vocoder or wah pedal because its spectrum contains one line only.
That's why I think that playing the funky wah-wakka-wah guitar is a very specific skill. It's not so much a matter of having the right stomp box but of mastering the skill of playing really good rhythm guitar, using dampening and working the wah-pedal together in the right way. It can then produce really curious effects.
I still think that the theme from "Shaft" is the golden standard of wah-wakka-wah guitar playing. Made in 1971, it is quite a bit earlier than "Le Freak" from Chic.