From what you've said it's not clear whether the amp is suffering from a momentary power interruption or whether the amp's on/off line (driven by the Riocar) is causing this.

If your windshield wipers have a bolted-to-the-chassis earth rather than a wired earth then I'd say it's more likely that the cause is related to the power feed somewhere. If they use a wired earth then the opposite doesn't necessarily apply

Where are you getting power and ground for the Riocar and the amp from?