Possible cause

WARNING: This page is about a different car, the 2016 Mazda CX-5. However, it is still accessible from the selected car via links, so may be relevant.

Connector terminal disconnection, poor contact, damage, deformation, corrosion
Audio amplifier power supply voltage or body ground malfunction
Open circuit in wiring harness between audio amplifier and connectors C-49, C-50
Connectors C-49, C-50 malfunction
Audio amplifier malfunction