So just to let you guys know, I believe I solved the issue. Have a look at this video to see what I mean: IMG_1423 | Allie Oop | Flickr.
So, there was no polarity diode connected during the video. All I did what add in the DC-to-DC converter :|. So, I’m starting to wonder if FWR is the true problem that caused the light plate to not work in the beginning. The light plate is definitely older, from like the late-70s to early-80s at least. Pretty interesting though, especially the part with the “Short” trouble appearing on the fire alarm panel.
I honestly don’t know if FWR is really the issue or not, but I’m just happy the light plate works again :).