if i remember my vehicles right it had the pos go to a aux terminal then to the one by the fuse panel. dont remember the neg but i would take both sides off of all of the big ones, use some heavy sand paper, or if you got it one of those wire brushes that fit in a drill to all the terminals and the metal or stud they mount to. has to be shiny on both sides, and put back together with dielectric grease or painted after assembly. then if that doesnt do it have to start moving on to the smaller grounds like the ones that go to the chassis for lights, pcm, etc.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock