Where is the starter solenoid located. Firewall? Can not locate

The starter solenoid is located right on top of the starter. It is the round unit with the wires connected to it.
The arrow in the sketch is pointing right at it.

Before you start looking around or working near it, disconnect the battery ground cable.
If you don't and something comes in contact with an electrical connector post (like a screwdriver, open end tool, or whatever) ... you will know something went totally and completely wrong "Guaranteed"!

You also could possibly get seriously injured so "DISCONNECT" the batter ground cable.

WARNING: When performing maintenance on the starting system be aware that the heavy gauge leads are connected directly to the battery. Make sure the protective caps are in place when maintenance is completed.

I hope this information helped;
Rick