Seasonal textures plus nightmap


I have an object and one of the texture, has seasonal condition but I want to add a nightmap as well. I've tried everything but the nightmap does not show up. He is using the summer texture.
I've tried to add the nightmap in gmax and the seasons with mdltweaker, it is not working.
As this is a FS2004 object there is no asm.
Any idea?