I don't remember exactly how to do that ( i've done it in the past but it's far...) If you have a texture on the mesh ( and i think that you have one ) you must create a night version of the texture ( if your daylight texture is mytexture.dds, your nightlight texture will have for name mytexture_n.dds ), put it in the texture folder, register your day version and your night version, not sûre for the night version, in the cfg base.cfg ( config folder ).
Edit:
Dont remember if the daylight texture must be in the cfg of your base under MESH as ownmaterial or named explicitly: TEX mytexture ( i'm not sure that that word, explicitly, is like that in english ).