Thanks all for the replies. I find it amusing that this detail is not in the PET diagram as there seems to be mixed answers.
I decided to look at the passenger side of my car and it has Pan head sheet metal screws going through the spacers and into the firewall. The firewall seems to have "hat section" sheet metal with a "nut" on the back side.
This earns a vote for a sheet metal screw.
By removing the spacer I can poke my inspection camera through the vinyl and see what is up on the drivers side. Interestingly there are the tips of TWO broken taps. One in each location. This leave me to think a previous owner was trying to clean up some threads.
This owns a vote for an M6 machine screw.
It looks like my project of finding the correct fastener is turning into a significant one.