Car Audio: Ground Hum While the Engine is Running

Hi,

I'm getting an install completed in phases. Last night the first phase was completed with two amps (1 mono, 1 4ch) and components in the rear deck. All the speakers are now connected to the 4ch amp. Thursday the box will be finished and the sub will be installed. Right now there's a problem. When the engine is running there is a distinct hum from the rear speakers. At first I thought it was a vibration issue, but every bit of plastic is firmly in place and the speakers mount to MDF. The frequency or volume does not increase by revving the engine. With the motor off, the speakers are dead silent. Any ideas? Any questions I should ask the installers?

Please help.

I didn't find the right solution from the Internet.

References:-
https://arstechnica.com/civis/viewtopic.php?f=6&t=252413
https://jobs.vidzzy.com/hire-creative-video-production-service/

Thanks