Problem with textures in shaders

[Dzsozi (h03)]

Hello! This is my first time working with shaders and I bumped into a problem. I would like to define two textures in one shader file, so I need less coding in lua and don't have to create the shader two times. I want to make a back-up light system for vehicles and I want to reach that by changing the vehiclelights128 and vehiclelightson128 textures. Since these are two different textures, I must replace them both when backing up with a car, since I can't detect if the lights are on or off if the setVehicleOverrideLights is set to 0 (default, changing automatically). That's why I would like to create something like onTexture and offTexture inside the shader file, but I don't know how can I do that, since I have never worked with shaders before. I tried something like this, but this doesn't work:

Shader file:

texture onTexture;
texture offTexture;

technique hello
{
    pass P0
    {
        Texture[0] = onTexture;
        Texture[1] = offTexture;
    }
}

I don't know how I should do this.

And the lua file (only a part of it, the backing mechanism and everything works):

		if not shaderReverse and seat == 0 then
			shaderReverse = dxCreateShader ( "files/texture.fx" )
			textureReverse = dxCreateTexture ( "files/images/vehiclelightsreverse128.png", "dxt5" )
			textureReverseOn = dxCreateTexture ( "files/images/vehiclelightsonreverse128.png", "dxt5" )
		end
		
		if shaderReverse then
			engineRemoveShaderFromWorldTexture ( shaderReverse, "vehiclelights128", vehicle )
			engineRemoveShaderFromWorldTexture ( shaderReverse, "vehiclelightson128", vehicle )
		end
		
		if engine then
			if getControlState("brake_reverse") and gear == 0 and getVehicleSpeed(vehicle) > 0 then
				dxSetShaderValue ( shaderReverse, "onTexture", textureReverseOn )
				engineApplyShaderToWorldTexture ( shaderReverse, "vehiclelightson128", vehicle )
				
				dxSetShaderValue ( shaderReverse, "offTexture", textureReverse )
				engineApplyShaderToWorldTexture ( shaderReverse, "vehiclelights128", vehicle )
			end
		end

So what doesn't work: the textures get replaced immediately on resource start, it worked when I did it with one texture in the shader file, but then I couldn't change the vehiclelightson128 texture, that's why I want to "define" two textures in the shader file. Hope you understand me. Thank you for your help in advance!

[Dzsozi (h03)]

Bump.