I figured it out, it is a material problem. But here I am stuck again... for this basic object of only 2 faces (child of fuselage, btw) I used the Completelyclear.TGA from fakepilot. In Dx9 it was really ok, it worked. Now it either fails to show up, or if I change opacity-diffuse whatever settings, it becomes a black rectangle still without decals showing up.
I tried to change the material to the semi-transparent material the cockpit uses, and voila, the decals appear on it. but of course it is semi transparent so the rectangle object is not invisible anymore.
I double checked material setting... nothing. Swapped materials... same result.
I figured it out, the the TGA files being used are somehow different. with the same settings, the completelyclear TGA is not shown as cockpit glass, at all. The crystal.TGA used as cockpit glass works like I mentioned above, semi-transparent. I loaded Photoshop to check whether it needs Alpha settings changed,but both TGAs were set up the same way...
What is different? How to make a properly working transparent or semi-transparent material?