Start by giving details: What year GLA, what market, what options.
Once you do that, people will tell you where the amp is. If you have a 2017-2019, it will be in the trunk, in the spare wheel well.
Go there and remove the amp. Read the part number, it is printed small right after a MB symbol. If you're not sure, post a picture of what you've read, for confirmation.
Then take that part number and google prices.
Buy it.
Try plugging in.
If a reprogram is necessary, go to dealer and ask for reprogram only, probably 1 hour at dealer billing rate.