Sounds like you also have power and ground reversed at the gauge too..
Hmmm.... Let's see... the original system was 6v positive ground , but everything is now 12 v. neg ground. The gauge is self grounding to the housing I think as there is no ground wire to the gauge ... which only has 2 connections... one is power ( now 6v) and the other wire goes to the sender.
Could I possibly have the sender power wires mixed? (Actually I don't think so as there is a metal plate that goes from gauge to gauge to carry power to the next gauge .