Oddball rheostat search

To make a short story long, I'm looking for a 12 volt rheostat for a motorcycle application.

I have a Gerbing electric jacket liner, works like an electric blanket, only it's powered by the battery of the bike, 12 volts. It draws 77 watts at maximum and is fused for 10 amps.

I made a coiled on/off power cord for it, but would like to vary the temp instead of all or nothing. I can just put it in line after the switch, or eliminate the switch altogether. It's for a bike, so weather resistence would be a plus.So, if anyone knows where I can get such a thing, I'd appreciate it.  

Thanks in advance,

Rich