Mine has an aftermarker unit. I gets warm, but not excessive heat. As said, they contain an amplifier with heat sinks on the back. They dissipate heat from the amplifiers output transistors, and that's normal.
 
How hot is hot? Hand hot is fine. Too hot to touch is too hot.
I took it to the car audio place near me. I had all the wiring correct apart from one of speakers that was out of phase. He felt how hot it was and though it was normal for the heat sink to get pretty warm.
It's not boiling but after about 5 seconds you would want to move your fingers away.
 

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