most VHF radios these days have a pair of wires for an external speaker, if these wires cross the volume output is seriously reduced, there are other bare wires coming that left alone can cause other issues. We deal with fishermen alot on these things and of course they know more than anyone, but are notoriously lazy when installing stuff.

We also encounter lots of voltage issues, related to bad wiring and poor grounding, in most cases these are attributed to problems with the unit not the electrical system. We power them up here and they work flawlessly. I would say that about half of the products we send back to the manufacturer come back with an "unable to replicate problem" message.

I think I've posted this guys blog before, I love his observations on all sorts of installations.
http://themarineinstallersrant.blogspot.com/