It's important to note whether the solenoid clicks loudly but the starter won't turn, or whether the solenoid won't click at all. If the solenoid clicks loudly then the heavy voltage from the battery isn't getting to the starter windings due to poor battery, bad cables bad connections or the starter brushes are shot or the windings are heat soaked.
It's also possible that the solenoid internal contacts are for the main cable are worn or corroded.
If the solenoid isn't clicking then a screwdriver jumping to the purple wire from the solenoid battery cable connection might make it work. If that's the case then there is a workaround using a Ford solenoid could be a cheaper fix.
Here's a tutorial on how GM starters work put together by yours truly for your reading pleasure.
http://67-72chevytrucks.com/vboard/s...d.php?t=655903