Just circling back. I think I found the problem.
I had installed a double din head unit a while back, and mistakenly powered the OEM amp from the following wire in the radio wiring harness:
Red/Black (thick) Constant power 12v (Fuse D8)
I've now moved it to this wire:
Orange 12 volt switched power (Fuse E1)
Fingers crossed that's the end of this issue. Thanks all for the amazing help here and the
radio wiring chart found here.