Tackle one thing at a time, it's not unlikely that the three problems are not related. I think GT-1 actually refers to the BMW tool that runs DIS and some other software which is what you are looking for. You will need a laptop to run it and if the package includes the WDS wiring diagrams it could be a huge help with the electrical problems.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)