W126 A/C vacuum line supply etc.

Have '88 420 SEL.

This weekend, got my blower fan back in high-speed order (required only bending one of the contact points back).

blows almost cold. BUT:

I am getting only some air from center vent. Here's what happens:

When the dash position in A/C is on, get some air from center vent, but still most from defrost section. When bi-level switch pushed, reverts to only top and bottom and absolutely no center vent. So----I surmise the damper is working to some degree.

I believe that I either have a slightly stuck diaphram door controlling center vent, or a loose vacuum line. The main vacuum line evident in engine compartment seems fine. So I guess I need some guidance here.

Any quick tips, without removing the dash (last resort)? Can I check vacuum line beyond the firewall or inside compartment at a typically weak point? Is there such a point?

Can I gerrymander or try to unstick a possible weak diaphram door reglating to center vent?

Can I get/download a diagram of the A/C vacuum lines for a 126?

Any educated assistance I can get on the above would be so much appreciated!!

Thanks.