Use this shader and the 2 materials I'm posting for you, it should give a nice dirty looking glass effect:
textures/amph_materials/glass_dirt
{
	qer_editorimage textures/amph_materials/glass_dirt.tga
	surfaceparm trans
	surfaceparm lightfilter
	cull disable
	{
		map textures/amph_materials/glassfx.tga
		blendfunc add
		tcMod scale 0.5 0.5
		tcGen environment 
	}
	{
		map textures/amph_materials/glass_dirt.tga
		blendfunc blend
		alphaGen wave sin 0.3 0 0 0 
	}
	{
		map $lightmap 
		blendfunc filter
	}
}
Obviously change the paths to whereever you want the textures.
Save these as:
glass_dirt.jpg

glassfx.jpg

I'll get them in the texture pack I'm doing and get them all GPL'd for you.