Well.. look how they brought in Garth.. no one knew him.. just all of a sudden they said he was a bad guy who is Wilton's son... all they would have to say is that KARR was KITT's prototype that was believed to be destroyed by KITT and Michael so long ago... etc...

Well, as someone said they would have to update technology.. what about having the guy that deactivated KARR to steal his CPU from the Foundation's vault and rebuild KARR better than ever? That wouldn't take long to explain...

And remember, when they took out his CPU, they put into a TV set!! Lol.. so it couldn't need too much to keep it going.