1998 Chevy Silverado Running lights

Hi, My running lights stated blowing fuses, this also kills the dash lights. At first it only blew when it rained. So common sence said check trailer light pig tail. I cleaned up conections and it was ok for about amonth, even in rain then it started blowing again. We realy went over wires for trailer plug. As soon as you turn on the lights it blows. My neibor said his had the same trouble and he had to replace the tail light assembly. I replaced the ciruit boards, housings and bulbs all at once. Got all done IT STILL BLEW THE FUSE! I thought I had figured it out for sure cause the one circuit board was corroded. I am at a complete loss. I hate to take it into the shop it could run up a large bill while they try to find the problem. I've been out of work 3 months so I can't do that.
Please any suggestions or ideas.