The speaker is at the right edge of the red circle. If you look through the rear window while you are feeling inside that hole, you can feel the bottom of the grill tab (off to the left on mine). Pushing on the tab will let you see the grill move through the rear window. Alternatively, you can lay down in the trunk and look up there with a flash light and see the bottom of the grill. That hole is directly under the inner (center of car, closest to 3rd brake light) rear corner of the speaker grill.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E