Make sure to use di-electric grease on the connectors, it will provide some weatherproofing. I basically packed the cigarette lighter output with this grease prior to plugging in the usb adapter, worked like a charm. I built my own cable that connects directly to the battery tender plugin, stuck a fuse on it. I left a switch off as it draws very little from the battery when left plugged in. If i want it off i just unplug it from the battery tender cable and voila.
Think everything cost me 15 bucks from radio shack, give or take.
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