The starter is just a motor that requires a main ground (usually chassis ground) and a hot lead to energize the wound coils and pull the bendix drive gear to engage. The solenoid is simply the switch. Jump the starter directly or remove it and bench test it or have it tested. Bypass your solenoid all together. Chevrolet's had the same problem cooking the starter mounted solenoid (and starter) when headers where added (small block 350/305)