'93 T-Bird starter solenoid problems

Something keeps shorting out my starter solenoid. I've changed it, changed my starter, and repaired the cables that connect to my batteries (they were a bit rusted). Nothing is working. It'll work for about a week or two then short out again. And if I disconnect it and reconnect it, it will start normally for a day or two. I need to know what could be causing this. And how I can fix it without paying too much.

Thank you!! :?