That link doesn't show a resistor.
The easiest way is to do a parallel/series switch, then you don't need a resistor, but it does make the "low" setting a good bit lower than otherwise
(25% of the high).
It all depends on how powerful the gloves are. If we assume 25 watt gloves,
12.5 volts from the bike times the current drawn = wattage of the gloves (25w), so the current drawn must be 2.
Ohms law tells us the 12.5V divided by 2 amps of current is a resistance of 6.25 ohms.
For 50% power (12.5w), we want only 1 amp to pass through the gloves at 12.5 volts.
With a 2.5 ohm resistor, the total circuit resistance is 8.8 ohms
This draws 1.42 amps
since the gloves make up 71.5% of the resistance, they dissipate 71.5% of the 17.6W total circuit power, so the gloves will run at ~12.7 watts, or half the power as without a resistor. The other 5 watts is wasted as heat in the resistor, so make sure your resistor is rated for at least 5 watts.
This is a pretty inefficient way of doing things. Using a 555 chip and a power mosfet would be much more efficient.
You could also just buy one of these for 23 cents, which would give simply run your gloves at 10 watts.
http://www.mouser.com/ProductDetail/...xYmnKIvA6xE%3d