Diagnosed rear passenger leak on unlock side, replaced vacuum element. Extremely satisfying to have a system that held vacuum for >36 hours and having the doors unlock Monday morning! Also diagnosed a leaky hatch element (300TD) on the lock side--as it's no longer available as a new replacement, I'll be writing a repair guide once a few parts get here. It's the newer style but Bosch makes some nifty caps that can repair a leak on the lock side of the system. I just capped the hatch/fuel door branch of the vacuum system for now to prove that everything else will hold vacuum. Thanks for the guide!
I can personally attest to the dissimilar metal issue: previous owner of my 300TD had recently spent several hundred dollars replacing window motors and switches and the windows still worked intermittently when I got it. I replaced the aluminum fuses and haven't had a single issue since!