Re: No Lights or Horn
I'm not a mechanic, I'm not an electrician and I certainly am no expert but I would go a slightly different route than what most have suggested. Actually it is part of what others have suggested but just a different aproach.
In the past and I have been boating for 50 years, every time I had issues with multiple electrical accessarys fixing one at a time was like pissing into the wind. I'd fix one item and something else would quit working. I found that more times than not the problem was due to a bad ground and if one is corroded there is a good chance they all are.
Here is my solution and like I said I am no expert. Go to your local marine store and spend $10 to $20 on a new bus bar to match the one under your dash. Now disconect every single ground from the old one, clean them up, put new connectors on the ends and connect them to the new bus. If that doesn't do it then proceed with the rest of the wiring. Once again I am not an expert, just relaying what has worked for me.
It won't cost you more than about $25, shouldn't take much more than an hour of your time and even if it isn't your problem it probably needs to be done anyway. In my opinion 50% of all electrical problems are due to a bad ground and this way you are assuring all of your grounds are good at one time.