Question: Did you coat the edges of the pads? That is where most of the friction occurs during braking as it rides against the calipers. True they say to put some on the back where the piston rides but there are anti-squeal plates available for that purpose and will probably last much longer.
I always do the edge treatment like this:
First, while the pads are out I run a small round wire brush in there to remove all the brake dust that accumulates (avoid hitting the piston seals). A wire brush designed for use in preparing 1/2 in copper pipes before soldering fits nicely.
Then dab some anti-sieze on the top and bottom edges of the backing plates (wear eye protection and do NOT run your eyes until you are free of the stuff!).
Also slightly bevelling the very edge of the brake pads using a grinding wheel or a flat file seems to reduce the squealing some pads make. Rears always seem to squeal - especially with Pagid and some Metal Masters (the latter should be banned!)

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!