Stereo won't power on, no volts on yellow

Might want to ask Swatguy via PM for his opinion. He might tell you to run a brand new, fused (he told me to buy things from Knu Concepts), proper gauge power source to a small buss you can use for the stereo and any amps in need of clean power. He's also probably tell you to run a fresh ground. I would not know what to tell you as I'm no expert here.

Have you traced the "yellow" wire from the unit all the way to its source? Experience with other boats, and recent experience with my 95, has taught me that even for a factory installed unit the colors can change at any time under the dash.

Don't know about your radio model, but with the units I've installed there are 2 ways they are usually designed to power on. One way the red is the 12v power that runs the unit, the yellow is a trigger of sorts that usually leads to the ignition or an ignition source, that way it will only turn on when the ignition is on. With the other the red is connected to the ignition for power - the yellow is power for the memory functions only. I'm guessing yours is closer to the first. I would google the brand and model to see if you can find an original installation manual for confirmation on exactly where this wire is supposed to be getting it's power and trace it from there.