Greetings all: I've owned a '78 E250 for the past 33 years so I know the truck pretty well. Over the years, corrosion has taken its toll to the body and electrical system. I finally got around to replacing all four light sockets, cleaned all ground connections, replaced the light switch, and turn flasher. The lights are working OK until I turn the head/running lights ON. When the switch is pulled, the brake lights and turn signals don't work but illuminate very brightly. Both filaments in the 1157 bulbs are on. What am I missing?

I did a forum search for similiar problems but after seeing hundreds of near misses, I decided to start a new thread.