I dont know about on the boats that use GPS, but a few years prior to yours, the signal wire from the triducer is the green wire. Check to make sure there's not a green wire loose at the back of your guages anywhere, and you can also test the green wire for voltage. I'm trouble shooting my speedometer right now as well. Here's a thread from the supra forums that may be of some help. Sounds like you have a bit different set up though with GPS speedometer. Do you have PerfectPass on it? If so, which PP do you have?