Question about Mp____.bmp graphics

[XLVIII Pz. Korp]

I have been experimenting with creating a graphic for loaded Soviet Trucks (up00044.bmp). I was able to create an Mp00044.bmp which actually works when the trucks are loaded. This works great for non-snow scenarios. My question arose when I took the next step and did the same for snow scenarios. Using loaded motorcycles as a guide (S-Mp00050.bmp) I created an S-Mp00044.bmp graphic for a loaded Soviet truck and placed it in the Special folder where the S-Mp00050.bmp graphic is found. Problem is in winter scenario's the loaded truck still appears as a regular non-snow Mp00044.bmp, and even more confusing when I unload them the unloaded trucks display using the correct S-Up00044.bmp icon. So how can I get my newly created S-Mp00044.bmp icon to display in winter scenarios? Thanks for any help.