Its in an enclosed environment and will generally collect some heat..how much?Who is to say? I have installed a mini amp just under the dash to the dash poll/bracket with Zip ties before without any issues..Out of the way, easy to get too to tune, and gets plenty of air
Depends which amp and how you're running it. My 6 channel sits enclosed under the back seat of the Jeep and has never had heat issues It has zero ventilation or space around it. Then again, it's built efficiently and I run it at 4 ohms per channel.
If you are concerned about the heat, and don't want to cut holes in the rear of your glove box; get a bracket to install it under the dash or front seat.