Need a little imput please. I'm putting six marker lights around my toolbox. One of the mounting screws grounds the light to the toolnox then I grounded the toolbox to the bed with a self tapping screw to the bed. I wired them using the 18 gauge that they came with to my fuse box just like my cablights are. Turned the key on and within seconds melted the wires. If I use a larger gauge wire like a 10 or 12 should that fix my issue also I plan to splice into my corner marker lights instead of my fuse box

 

I assume that they are not LED marker lights if it pulled enough to melt wire. First of all never run power from your fuse box, there just isn't enough there to handle the load. The wire size should be adequate for no longer of a run or draw from the lights, you probably just need a better ground for your system. One self tapping screw from the tool box to the pickup may not be enough. Just like trailer lights, you're better to run true ground between the two vehicles.

 

Are they LED? Post a picture.

Melted/heated wires means bad connectivity or high resistance. Did you twist them together or solder? 18AWG is fine for LEDs, and while I wouldn't do it, the fuse box is plenty of power for LEDs since they only draw a tenth of an amp. But 10/12AWG is simply TOO big. 18 is fine, just find you problem and wire them somewhere else.