I had a similar problem in my 81 El Camino,
the problem was a bitch to find, turned out, it was a shorted marker light
bulb socket. This puppy defied any realistic test, pull the light switch,
got power, signal for turn, got power. when I cut the wires, the problem
dissappeared, a new socket fixed it. The socket looked ok, but was smashed,
and shorted . A shorted socket allowed left turn signal impulses to travel
through entire running light circut, causing all lights to blink.