Motorcycles cigarette lighter keeps blowing fuses in the charger.?

I'm having some difficulties. I hooked up a weather proof 12v to my motorcycle (yamaha r6 2008) I made sure to put a switch in it so I could turn it on and off, but no fuse. The charger that I put in it has a 1.5amp fuse that comes with it. The charger will work temporarly when I put a new fuse in it but then blows shortly after. There is no short and I took extra care into wiring. I have also put a multimeter on the batter and the 12v both which are putting out the correct voltage and amps. I'm going to put a new switch in it to make sure its not the switch . But I would think that if it was the switch it world be instantaneous blowing of the fuse. Multimeter reader were 12.64ish and amps 6mamps. I have a feeling that its the alternate causing it because its once the bike has been on for a bit, so of you all concur then where do I wire into?