300 SE Shifting hard due to A/C Vacuum Leak?

'90 300 SE was jerking pretty hard during shifts, so I took it in for transmission service--had been a while anyway, so why not...

Shop did the work and when I went to pick the car up, they said that the car would shift smoother with the A/C off because of the vacuum leak in the A/C system.

Hmmm....really...?

I asked if they could fix the vacuum leak. "Well sure, but it is likely in the dashboard like these leaks usually are"

Hmmm....you don't say....?

So my question is this--Is this "typical A/C vacuum leak in the dashboard" a myth? If not, is it something I have a reasonable chance of finding and fixing, or is this pretty difficult?

How do I get started--such as how does the dashboard come off? (I'm assuming it is the section with the AC controls on it... )

This is a great forum--much help already delivered...