About the "screen" and "multiply" blend mode...

Both blending modes (multiply and screen) always take the pixel values of the layer underneath in consideration. So, i.e. when you set the top layer to multiply over the bottom layer, as in your example with the red car over yellow background, everything that is NOT solid red on your top layer will show through.
I am assuming you want the red car only to be red, so the easy way could be to set the car layer's blending mode to normal and mask out the background via layer mask or simply erase the car's background.
Hope that helps.