PLEASE DON'T DO THAT!!!!
There is nothing worse/more dangerous on a boat than spagetti wiring. Doing things right, so that you can deal with eletrical issues quickly when your out on the water, is easy if you just spend a little time. With spagetti wiring, things just tapped into other things, if you ever have a problem it is going to take hours/days to repair. Also if you ever have an eletrical fire you are not going to know/be able to de-energize the circuit. It does not cost a lot of money or take much time to set a boats electrical up right.
If your boat already has a grounding block and a DC fuse panel adding your radio is as simple as running the negative wire from the radio to the grounding block and the positive wire to either an unused slot on your fuse panel or tieing it into an existing fuse.
Your boat should have a cable running from the negative battery terminal to a grounding block under the dash. EVERY electrical device on the boat should have an independent ground wire running back to that block. You'll be able to tell the ground block by lots of wires running to it with lots of little screws in it. On my boats I have a grounding block in the stern and one in the bow.
On the postive side you should have a fused DC distribution panel even on small boats. EVERY electrical appliance on your boat should run through that panel. Using this system every electical appliance on the boat is fused and is easy to energize/deenergize. It's okay to gang items under one switch/fuse. For example on one of the boats I have two VHF radios and my stereo under one switch labeled "radios". With your boat set up this way and a well labelled fuse panel ANY electrical problem can be diagnosed and fixed in minutes.
If your boat does not have a ground block or DC fuse panel you can install them in less than a day and for only a few hundred dollars with minimal tools. It's money very well spent. If your boat does not have a ground block or fuse panel and you want to install one just let me know and I'll zing you a few suggestions on how to set them up and install them. For a few extra bucks I'd strongly recommend installing a batter isolation switch while you are at it is there is not already one installed.
To save me getting emails I'll note that the exception to the above statements is that your bilge pump, dowriggers and trap puller should be wired directly to the battery in the stern and not run all the way up to the bow distribution panel. This is why I have stern grounding blocks and small panels for clustering my bilge/downrigger in-line fuses in the battery compartments of our boats.
PS you can't use your VHF antenna, try without or install one of the short 8-12" AM/FM stub antennas.
Good wiring makes for trouble free boating!!!!!!!
