Should have a purple wire and thicker red wire with fusible link on it at the starter.
Purple is ignition to solenoid. Its hot only in start position.
Comes off firewall and drops down at rear of passenger side of the block
Red is to junction block which feeds entire truck.
The red one you replaced needs some kind of protection on it due to 12v constant and if battery or alternator has melt down, there is nothing to stop voltage from rest of the wiring.
Where is your alternator charge wire going to? Straight to battery?
Check the ignition switch connection under the dash.
Check the neutral safety switch.
If it checks good, go to the purple wire on solenoid.
Use test light to check for voltage when key is turned to start.
No power there, trace it back look for breaks or melted wiring.
If you do get voltage on start position, then test the starter solenoid.
Here's a diagram.
http://www.73-87chevytrucks.com/tech..._V8_engine.jpg
let me know how you make out.