Proper way to install polarized plug

[ldatlof]

I have been reading up on how to install a polarized plug and I am not sure exactly what to do. I have attached (I had to use a zip to make it small enough) a drawing of how the radio is now wired. The bottom picture is how I think the instructions say to wire a polarized plug so the chassis will always be grounded. The red line is hook up wire to be added by me. Please advise. Thanks.