Installing reproduction side mirror

[c10boob]

I ordered a reproduction side mirror (driver side) for my 71 so it will pass inspection. I got the mirror from Classic Parts of America, and it came with the plastic gasket for the outside as well as the metal bracket for the inside.

The previous owner of my truck covered the original holes up (no big deal).

So my question is how do I install the inside bracket to hold the mirror in place? I took the door panel off the inside but it doesn't look like there is a way to get to where the bracket needs to go, because there is still metal covering that area.

Anyone else ever encountered this and came up with a solution? I'm guessing it's something so simple I'll feel stupid after hearing it - but I'm just not seeing a good way to do this.