Under Dash Speaker mounting?

[McCoyAZ]

My old '64 never had tunes and it's time to install an under dash speaker.

I bought what was supposed to be a "direct replacement" stereo speaker from Brothers. No installation instructions, or diagrams of any kind were included.

Looking under my dash, which for an old fat man with a bad back is no simple task, I see the grill where the speaker would mount along with 2 unused vent holes.

The speaker I purchased has 2 Z shaped brackets attached to it as if they were to be secured someplace, but the roadmap to that someplace wasn't included.

Here is what I have..... all I need to know from someone that may of been in the same boat as I, is some direction....where or how is this to be mounted?

Thanks in advance.