2003 GMC Sonoma left side headlight, signals, taillight

Electrical problem
2003 GMC Sonoma 6 cyl Four Wheel Drive Automatic

Well to begin with about a year ago I put a plug n play wire harness for a trailer on my truck.(No cutting or splicing). Just a short time ago when I turn my left signal on, my dash and stereo lights blink along with it. This also is happening when I tap the brake pedal. My left tail light does not work unless I turn the switch"on", on the dash. But when I do this my left signl does not work and the arrow indicator stays on with out blinking.