When iPhone 12/13 arrived and Apple moved to MagSafe charging, the position of the charging coil on the back of iPhone was moved to the exact middle of the phone. This is likely an issue of the charging coil in the charging pad in your car not aligning properly with the charging coil on your iPhone. Mercedes designed the pad in your car before Apple repositioned the coils on iPhone and it's almost assuredly an alignment issue. There really isn't much you can do here. I too own a Mercedes designed before MagSafe charging was engineered into iPhone and my iPhone doesn't charge properly on the pad. So, I have a Weather Tech Cup Holder for my iPhone and I plug it in to charge and use Apple Car Play at the same time.