KW Electrical problems

2000 T2000 cummins N14

Having electrical issue. she's a great truck and even though I have a few others, I'd rather try to fix her than get rid of her. I think i had a bad inverter in the lighter. Been having issues with gauges. I shut her off and she wouldn't start. Had to tow her to the garage and they said it's the harness but they also cheated me on labor costs, putting parts on I didn't need, then refusing the parts waurentee, I could go on, but I'll spare ya'all. In the end they couldn't find the part to fix it so they got it started but told me I can't shut it off.

I'm just wondering if anyone has any tips on tools I might need to get in there and track the source of the issue myself? Thanks!
(Obviously I'm not a mechanic or electrition.)