Installing bilge blower

[Welti]

I have a 25 foot sailboat with permanently installed fuel tank under the cockpit. I would like to install a small blower under there as a safety precaution. I have all the parts, just trying to figure out where exactly to tap in to the electrical system to get power. The fan draws 4 amps, not that big. There is a wall-mounted positive and negative terminal next to the batteries that the batts are connected to. Everything else is connected to it. It is a heavy duty bolt with plastic cover which partially covers it but allows wiring access. There is also a terminal block next to the switchbox (lights electronics autopilot etc). Only issue is that that terminal block is really hard to access. In fact, everything is kind of hard to access, it is extremely cramped in there! I could access the battery terminals ok though - not the actual batt terminal but the wall mounted one I already mentioned.

Any general suggestions on how to get my power? I'm tempted to just get a 12 v lantern battery and power it off that! I don't think it will be used often.