There is a small grille near the center rear view mirror. Hold some smoke near it and see if the smoke is drawn into this grille while the ignition switch is on.

If not, you need to find the small fan that draws the air sample through that grille. The location of this fan varies within the 124. Mine is in front of and to the right of the glove box. Remove the glove box and see if it's there. If it is not turning with the ignition on replace it.

Good luck,