1994 Mercury Topaz Stater

I own a Mercury Topaz 1994 witha 2.3L. I replaced the starter in January. About a month ago the starter would not disengage after the engine was running and kapt going even after I turn off the engine. After 30 seconds it stopped and worked properly. Since it would randomly engage, but only for a second. I replaced the key lock cylinder and the ignition switch on the steering colunm. I also swapped out the starter since it was still under warranty. This has not helped and it has gotten worse. The starter will engage anytime the battery is hooked up. Any thoughts? Thank you for your time.
Chris

updated 4-7-10
Thanks for getting back to me. I unplugged the red wire with the blue stripe and the starter kept going, so I bought a new solenoid, but the starter stills keeps going like before. I have now replaced the starter, the solenoid, the key lock cylinder, and the ignition switch. Is there anything else you can suggest I try. Thanks for your help.