First can he stand tied in an arena where he can watch another horse( a buddy horse if at all possible) being driven to the cart? Then Have someone driving the other horse in the cart and you longlining him directly behind the cart, then in an arena, you longline him while someone else is driving the cart put to the other horse, particularly one he knows well ?
Two things here, he sees and hears a buddy horse driving quietly to the cart, and longlining him behind the cart being driven is similar to a technique we have used to get riding horses who are afraid of carts or bicycles to get used to them, it has to do with the horse feels comfortable when they are the "chaser" and they are chasing the scary thing away.
After that I would consider ponying him behind the cart while driving a buddy horse in the cart.
He really could gain confidence from a trusted herd mate who is calmly working in the cart.
Two things here, he sees and hears a buddy horse driving quietly to the cart, and longlining him behind the cart being driven is similar to a technique we have used to get riding horses who are afraid of carts or bicycles to get used to them, it has to do with the horse feels comfortable when they are the "chaser" and they are chasing the scary thing away.
After that I would consider ponying him behind the cart while driving a buddy horse in the cart.
He really could gain confidence from a trusted herd mate who is calmly working in the cart.