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So, you have the electric wipers....
Your little part is called "tapered splined bushing" and you can get it here (just to name one of possible sources):
https://www.oshkoshequipment.com/products/detail/71/1607/Windshield-Wiper-Motor-Arm-Splined-Bushing
TM 9-2320-361-24P (Aug 2006), Fig. 173...