Location of OnStar box

I have a 2012 Impala bought used with 19K. I have an aftermarket OnStar unit installed and it had a small yellow triangle flashing. OnStar is activated and they ran diagnostics on the system. I was told the back up power was dead.

Does anyone have an idea where the aftermarket system is located in the vehicle? I want to replace the battery in it.

You had the Onstar system added to your car?

Was this within one year? If so, I would think it would be a manufacturer's warranty, involved here......so it should be going back to whoever installed it......I don't think this uses a plug and play battery....it might be part of the unit....In GM cars they are usually located behind the back seat, but aftermarket? Who knows?

Just bought the car. Will look under and behind the rear seat. Was installed before I bought it