NBC got Alvin & the Chipmunks off to a fast start by having another of their stars pay Alvin, Simon, & Theodore a visit in their opener. The network figured that the best way to promote the debut of Mr. T later that morning was to have the big fella make his toon debut on Alvin, coming to the aid of the boys when bullies steal a gold watch Alvin brought in for show & tell.
Edit, 4/22/18: It's back, coupled with the debut of "The Chipettes":
Ruby-Spears obtained licenses for both Mr. T & Alvin, but would lose the Chipmunk license to rival DIC a few years later. NBC would lose the Chipmunks altogether, as the series later moved to Fox to finish its run, albeit under a new title. But that's another story for another time.