I am installing a light by back door controleed by a switch .I purchased a plastic mount block. But I don't see how to mount electrical box behind block and siding nothing to screw to. Can someone point me in right direction?

How is for a set o finstructions:

Installing A Vinyl Siding J-Block For An Outdoor Electrical Outlet (http://www.hammerzone.com/archives/exterior/siding/vinyl/accessories/j_block_01/outlet_box.htm)

Here is another product:

http://www.aifittings.com/whnew41.htm

Your electrical distributer will have something as well to make the job easier.

And here: http://www.arlcatalog.com/

http://www.arlcatalog.com/Siding/Siding%20Box%20Kits.htm