That light is shipped with a 2s battery -- so it maxes out at 8.4 volts. In general, those lights will work fine on a 3s battery (though this is not 100% certain), so 12.6 volts, and a car system can go up to 13.8 volts. (Motorcycles too, right? No 6v business any more?)

If so ... then there may not be any modification needed at all. That said, you'll probably need to talk to the manufacturer to be sure, or just try it. (If you do try it, run it on high for an hour or two, outside in case things don't work well. Have a fan blowing on it to simulate riding -- they tend to get excessively hot with no airflow.)

Talking to the manufacturer may be difficult, and the seller probably doesn't know ...

If you do need to modify it, the easiest thing to do will be to use a DC to DC converter that accepts 11-13.8 volts and outputs 8 or 9 volts. Just make sure it's rated at say two or more amps output.

Also note that there's a bunch of different variations of that light, all approximately the same. You can get an almost identical light for around $15, though you won't get the free taillight at that price.