Yes, for FS2004 assign a diffuse texture that ends in _t, and then have an _L texture in the plane's folder. It will be used automatically.
Note that all of what I am talking about is for the MakeMDL compiler from MS; FSDS2 (for example) uses its own compiler and does not follow these rules.
The difference between night textures:
1. An _LM texture is a night version of the day texture, and will *replace* the day texture at night. The day texture is not displayed.
2. An _L texture is an overlay. It lays over the day texture, and the resulting display is the two textures *combined together*. So where the night texture is black, you will see a much darkened version of the day texture. A 0,0,0 black area in a night texture will provide the same darkness as if there were no night texture at all. Anything lighter than that (20,20,20) will give the dark areas a dim glow. Some people like the glow, others do not.
If you place a yellow/white square on the night texture, that area will be lit up (and should be in the same place as a window in the day texture). A light splash in the night texture will create a splash of light onto the day texture, like logo lights lighting up the rudder. No color or other information is required in the night texture (it can be simple black and white), but if you add a color it will be combined with the day texture (allowing you to create some special effects, or just a warm yellow light). You could even get fancy and put silhouettes of people's heads into the lit windows of the night texture (but some people don't like that).
Hope this helps,