Here is a link explaining how to add a back-up switch to an SM420. I had one of the factory kits, but like the set up described in the link much better.

http://s844.photobucket.com/user/ma...20PM_zpsfsnbzrj6.png.html?sort=2&o=0

Matt

Last edited by Maybellene; 11/15/2016 4:45 AM.