Blowing fuse when turn on running lights

Hey guys I have a 1970 Dodge Dart swinger I did a full nut and bolt restoration on. I have a new engine and chassis harness but had to use old harness under the dash. So ever since I have reinstalled my dash I have had a issue where the second I turn my headlight switch to the running lights position it instantly blows the fuse. If I continue to switch to the head light position I still have headlights. Once it blows the fuse i still have headlights but no side Marker lights and no rear running lights. All the brake lights and turn signal work properly just my running lights. Could it be the switch itself causing a problem? Or a wiring issue? Any help would be appreciated on how to trouble shoot it.