select float2 uv_Maintex to UV map channel in shader.

I need to select between multiple uv map channels from a single blender mesh. Only one channel needs to be used per material. All replies on question like this say it must be done by a custom shader. I think I only need one shader per uv map channel. How can I set float2 uv_Maintex to point to a specific blender UVmap channel?