Turn signal help!

Need some electrical advice here.
Working on taillights on 72 Duster. Driver side works fine, passenger blinker works UNTIL brake is applied then it stays solid on with just a hint of blinking. I can hear the flasher working under the dash.
If neither signal is on, brake lights work fine. If the right signal is on, the left brake light goes out and the right signal juuuusst barely blinks faintly. Front is same way.
WTF is going on here.
Oh yeah, replaced flasher with no change.