i went through the same thing as you twice!!! with two seprate problems.
you obviously know what wire goes through the firewall down to the starter, just hook a jumper wire onto that wire, then touch it to the battery hot. you will instantly know if the problem is before or after that wire you just jumped. when i did this, it cranked. so i started looking before that wire and found out i had a weak connection from the nuetral safety switch.
the other time was from a old ( cheap) battery. it had enough power to start sometimes, but when the car was warm, and required a little more amps, the battery just didn't have it.