electrical woes

[Jason71k10]

I'm trying to get the taillights working properly so I can put my truck back on the road...but it's not going so well
here's what's happening:

-all driving lights work fine

-Right turn signal works fine
- when I turn the left turn signal on, the ip lights(including tach and other added gauges) flash in sync with the t signals, also front right signal flashes, but not as bright

- when I press the brake pedal, the ip lights also come on
- also, when i press the b pedal, the taillights are only slightly brighter than when the lights are on

I guess it's a bad ground, but I took the ground strap off, and sanded around it but it didn't seem to make any difference.
Btw before I disconnected the rear harness when I removed the box last fall, everything worked fine. I have it all hooked up the way it was before, but still doesn't work properly.
Any thoughts guys?? Thanks
JAson