When I connect my light directly to my battery, it is at full brightness, but when I use a long cable to install it, the light is dim. What is wrong?

You need to make sure you are using an appropriately sized gauge wire for your installation. This is especially important if using a long wire from the battery to the light installation. If the wire gauge is too small, there will be significant voltage dropped across the wire, preventing the light from powering up fully.