I will appreciate some ones help:

I have a '96 E300D and the air flow on the A/C decreases when I hit the gas also I hear a whistling sound for the first few minutes till I start the A/C on.
I know it has do with Vaccum leaks but apparently cannot figure out where.
I checked under the hood and all vaccum pipes are intact.
If this is under the dashboard how do I open this and fix the problem.

Yes, vacuum leak. Don't know about Diesels, they may have a vacuum pump as well that could be failing. Get a manual. It could also be one of the vacuum-operated flap positioners that route the air to heater, defroster, or dash vents. Sometimes either the defroster or the heater vent is the default, so it routes the air there when vacuum gets low.
Anyway there is a procedure for troubleshooting the vacuum system in the manual All I have is a W126 manual, may be one on here. Help will be with you shortly, Patience Grasshopper.

Appreciate your response. Would you be able to provide me with a copy of the manual 126 you have?

How do I remove the dash on this car? Can I get the procedure?