Sounds like a good place to start.

The GP's are easy to check even while in the car. All you need is a multi-meter to test for ohms. If you don't have one let me know, I have an extra one you can have. I work in San Francisco a few times a month so Oakland isn't too far away.

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'98 E300 Turbo "Juliette"
'85 Federal 300TD