No, every lock has it's own vacuum line which work independently, all via the vacuum pump.
I had an air leak in my driver's side rear door and I could hear air hissing. It turned out to be the black rubbery thing (I think the actuator?). Yes, definitly listen for a hissing sound when it's struggling. But you have thick door panels and might not hear it as easy... more than likely you're going to need to take the door panel off anyway though... it's not that difficult. Keep us posted.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)