Whoa; hold on a minute. Don't think you want to do that either. There are mods on the internet that I've seen that shows how to bypass the amp. It uses a switch that will defeat the amp's 12v feed power. So you might look/find the article, post it here, and then ask the gurus if it is correct before doing anything. Anyway, that is what I would do and why I would do it that way.
I would do it this way - rather than hacking up the radio as you first planned.
If this is what you want to accomplish; then you do NOT want to put an IRF520N in your radio - either. You won't need to. If it has the 2sc1969 final transistor (probably); then this should drive your TS-667 as you want it to do/as it should be done.