I have nothing to add, except another data point–isn’t this the horn heard in one of the Betty Boop intros (at 36 seconds in)?
I’d also like to point out that coincidentally this thread not only concerns animated shorts, but is literally about a “car tune”.
Ah, thanks…I thought I had heard it in a Betty Boop cartoon, but I didn’t realize it was in the actual theme song! It wouldn’t surprise me if this “car tune” (as you call it) popped up in other Fleischer cartoons.
I seem to recall a black and white (Disney?) barnyard animals cartoon, in which an elongated duck or goose makes this Sparton horn sound, then all the smaller animals hop up onto its back as if boarding a bus. 