You have an optocoupled relay, the relay-switched power and ground are isolated from the controller so you can run completely separate power through the relay from your Nano.
You have them tied at VCC. When the motor starts and stops your Nano is going to get power fluctuations in line with power use.
The relay should be connected to car + and - to turn power ON and OFF to the wiper motor. The Nano must get clean 5V.