Where are the W124 instructions you found? I've only found instructions for the up to 85 PBUs which come apart easily. The bad solders in those units usually are where temp selector and fan speed buttons join the main board. 86-up PBUs are more... uh... integrated. I have the PBU from my SDL in front of me and I can't figure out how the board comes off either.

Sixto
95 S420
87 300SDL