You have problems that I can think of an easy common point other than a ground. Are YOU willing to perform the trouble shooting steps? Or are you just interested in someone giving you a "magic bullet" solution?
If you are willing to perform the trouble shooting steps yourself, then IMO it would be best to focus first on the most serious problem first. For example the brakes. Can you be more specific about exactly what the symptoms are?
Having a test light that can "load" the circuit to give accurate results. This is especially important when a ground problem is suspected.
Here's some information about voltage drop testing that if the method is understood can be used to test a ground's ability to carry current and how to use the voltage drop method to narrow down WHERE the problem might be.
Howto perform charging system voltage drop test
To anyone else reading any of my posts I usually include the line: Today's cars simply will not run right without a strong battery and charging system! Cut corners here at your own risk I'm a big believer in starting with the basics. As such I...
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There is a ground behind the center console that is shared by multiple devices including the cluster, GEM, radio and others. I could see a case where a bad GEM ground could cause problems to the GEM. Since all of the systems mentioned depend upon the GEM, do you see where this is leading?
Since you have already replaced the GEM once you should know how to access. Suggest using the voltage drop method or a test light to confirm the ability of the ground to carry a real current all the way back to battery negative. Don't use a VOM to perform a simple resistance measurement to test a ground. Resistance will not always uncover a weak ground.