The Dark Backward(1991)

Overview:

A man pursues stand-up comedy encouraged by his fellow garbage man. Though his friend, who accompanies him on accordion, continues to tell him how great he is, he actually stinks. When the "comedian" grows a third arm out of his back, the friend uses this twist to get him signed up with a sleazy talent agent, and it begins to look like his career is on the move, even though his girlfriend has left him.