Bess’s Brake lights and Turn Signals

Bess has been on a tear lately, but I won't go into all that right now. Here's my latest "challenge ":
Brake lights/turn signals work UNTIL I turn on the running lights/headlights. I did recently replace the headlight switch and connector due to China Syndrome. Any ideas?
Bad ground??
OK, OK. The flasher is functional, even with headlights on. The brake lights/turn signals all work. HOWEVER, with the lenses covers on and with headlights on, you cannot see brake lights/ turn signals. It's...

Bess's Brake lights and Turn Signals
Author: Mick