2000 GMC Jimmy Won't Start

I have a 2000 Jimmy and it just won't start. All lights come on and horn works. No crank when I turn the key. I had this problem before and the problem was a bad starter relay. I replaced the relay. The vehicle started up fine and ran fine for a few days. I went to start it this morning and it wouldn't. Battery is good, changed the plugs, wires, starter, cap and rotor within the last 8 months. Someone made a reference to my security system. Are there known electrical problems/interferences between security system and other systems?

Does the secuity lite stay on, have you checked for 12v at the solenoid when cranking?