Puddle map is controlled by Alpha Channel (opacity) of "Details Map" (roadDetailsMap, rdt, also this 397 guide). There are a few ways to do it with free/opensource software. In Blender, you can select all road mesh, and add a new UV channel to each of the road materials, then create a new texture & UV map (with all selected road mesh as a whole) in this UV channel, then paint puddles on this texture (puddles are the transparent part of the texture, you can also take reference from LochDrummond's rdt dds file). Once done, export (***_rdt.tga > MapConverter > ***_rdt.dds, also all the new *.gmt with new UV map) & loading track in devmode. Then open material editor, and assign ***_rdt.dds to Road Details Map, change UV channel to one of the channel that you added in Blender (depends on order, could be UV_B, UV_C, or UV_D). Then you can test wet effect with weather panel. Also another slight easier way to make puddles without having to map extra UV & texture. You can just create a black Road Details Map texture (size based on main road texture) and paint (transparent) puddles directly on it, then export and assign in material editor using UV_A channel (same as main road texture). If you do it right, it will look just as good as the other method.