What makes you think you can't install one?
Go to the Soundtraxx website (a Google search will find it), download the instruction for the decoder, read through the instructions. It does a thorough job of explain how to install the decoder. It will do a much better job of explaining the process that anyone here can.
Or, is there something specifically in the instructions you don't understand?
The Soundtraxx manual explains how to determine the stall current.