1990 Mustang GT

this might sound dumb but did you check to see if you connected everything to the solenoid right? i just got over the same problem.....turned out that my original solenoid was shot so I replaced it. I connected all the wiring just like I had taken it off. bad idea. turns out the + and - terminals were opposite on the one i bought from the parts store. check that, if you hooked it up backwards then chances are that solenoid is shot now.

Disconnect the wire or cable on the normally open solenoid terminal (the big lug with only one wire attached to it). This just keeps you from frying the starter due to overuse.

Now put a test light on that now-bare lug and ground the other end of the test light. Connect the battery. Does the test light illuminate? If so, remove the slide-on terminal at the top of the solenoid. If the light stays on, the solenoid would seem to be latched. If the test light went out, that suggests an issue in the starter interlock wiring.

very nice!