Multiply the number of LEDs in the light by 15 or 20. Most LEDs consume a maximum of 20mA, so my multiplying my 20, you will get the worst-case scenario current draw in mA. The voltage rating should be listed with each light, just multiply that voltage by your calculated current and you will find the amount of power it takes to run the lamp. Just make sure you don't mix up A and mA...

Also, are there any auto supply stores in your area? Most of them will probably have a selection of auxiliary LED tail lights for trucks and trailers. You can at least save on shipping this way, and you'll be able to hold the lamp in your hands BEFORE buying.