If you suspect grounding the best place to start is at the battery negative. From there all grounding terminates at the engine/transmission grounding stud which is located on the back of the engine (driver's, radiator side). There are also grounds located in the main engine compartment fuse box, the cabin fuse box and left side of the dashboard. In order to systematically check them you will need a GM Grand Prix service manual. All other reference sources are too generalized. Just about all of the ground wires are color coded black. All electrical components reference or require a ground in order for the circuits to work.
Also, your problem could be some component loading a respective circuit down or going South. About all you can do is visually inspect the grounds, make sure they are properly connected and tight. After that a wiring diagram and multimeter will probably be needed. With the proper documentation you should be able to troublesoot and see what is common to all the problems you are having.
I can't make any promises but I'll review the wiring diagrams and provide hopefully be able to provide a starting point.
BTW - Welcome to the forum.
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
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u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
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