I'll chime in on the rain bit, because I actively seek it whenever I can. Rain isn't really a big deal as far as making the roads overly slippery is concerned. In fact, heavy rain will make them considerably less slippery by washing away the oil buildup on the road. However, light rain can be a problem because the oil just sits on the top. The best thing to do is to take it a little easier. Take the corners slower and don't lean into them. Brake earlier for traffic lights and such to allow yourself an extra couple of seconds to slow down, and try to avoid slamming on the brakes if possible (you might consider riding slightly slower to facilitate this).
In answer to your light question, you might need to provide a little more information. What brand of light is it? What type of mounting system does it have?
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That is all.