Injectors will not cause idle to drop or deviate. I have seen where the engine harness will rub the intake and wear a hole in it. Every once in a while, it will hit and cause a low idle.
The center vents are for A/c only and should be turned off in the winter.
The heat comes out via vents under the seats.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS