If you have a 12v lighter socket with the correct polarity, you can plug a USB charger into that. You can find for about $5 USD some fairly strong lights on Ebay, shipped from asia (shipping can take a month) that plug into a USB port.

The one I have (photo below) when you plug it in, it initially is off, one push on the button and it is in high mode that runs at something like 6 watts, press the button again and you have about a 1 to 1.5 watt powered light. One problem with that is that if your power source is not strong enough to power the light on high (6 watt out of the USB port), then the light will not function and you can't even push the button a second time to get to low power. I have no clue if the 12v USB outlets for a lighter socket have that much power or not. But, it is cheap so if it does not work for you, you did not lose much money. The light I bought straps onto a handlebar or something round. It is a flashlight type of beam, circular, not low and flat like is best for a bike light.

If your handlebar bag is not too close to the tire, you might be able to mount something on the fork crown that you can strap the light too.