Allright, I just drew this out, let's try the KISS route first. If I could figure out how to post my drawing I would.
One way this is gonna work is to use a double pole relay, the solenoid goes on the second pole. This is the simpliest way of fixing this issue. Alternativly a second single pole relay would work, but to me that would clutter things.
The second way is with a diode. The diode would have to be installed after the relay, and after the connection to the solenoid. It has to be here, and now it's carrying all of the current of the fan. If I remember correctly, the fan draws on the order of 20 amps, so to have some room for heat, a 30 amp diode would be none too big, so Radio shack is out, they don't go that big, I just checked, you would have to use one of the big electronics suppliers,
www.newark.com, or
www.digikey.com to get a 30 amp 14 volt diode. You are also in the price range of a relay, 5 to 10 bucks.
One of the problems with this route is by the time you get a diode that big, you are into stud mounts and heat sinks with both sides of the heat sink being above ground electrically, which means that it needs to be isolated, no just fastening it to a handy piece of bracketry. Mounting it to the front of the fan might be an option, that way the air flow would keep it cool. I am assuming that it is a plastic fan and guard, no shorting issues.
Once a solenoid is pulled in you would be suprised at how little current is needed to keep them pulled in. On the order of milliamps.
Sorry to disappoint you Bill, but the fan isn't charging the battery, it's just lighting the light, there's no connection back to the battery with any of the switches being off. A diode in the line after the switch and indicator, heavy enough to carry the current of the fan, will prevent the light from coming on as the fan spins in the wind. Any diode in the line to the light, would keep the light off at all times.
Figuring out this kind of stuff is what I do for a living, so that's where I am coming from.
my money is on the two pole relay.