A friend of mine who owns a body shop told me the best way to do this is to add your tint color to clear coat spray. You can either add enough to get the darkness you like, or make the tint light in the clear coat and add layers to darken it as desired. The more layers, the darker it becomes. This is done to the outside of the tail lights.
To prep your tail lights, you need to lightly scuff them so the clear coat will adhere to them. Then clean it with acetone or Prepall cleaner. Once clean, add your first coat. Before the first coat dries completely add your next coat. What I mean by "before it dries" is, it should still be slightly tacky. Check the tackiness in an area which is inconspicuous. Add how many ever coats you're going to add. You may end up needing to sand/cut/polish just like it was a paint job, because in effect, it is a paint job. You should be able to purchase the supplies you'd need at any auto paint supply store, or if you tell a body shop what you want done, they could easily do it for you.
There is another alternative, but it doesn't leave you with the option for lighter/darker amounts with much adjustment. The product is called VHT Nite-Shades. It will go on much darker than you are considering, I would bet.