i would start here
http://outintheshop.com/faq/wiring/wiring.htm
you may need to click on a link when themsite comes
up.
most if not all the lights you are speaking of use the body itself as ground so id think if
they are working at all your ground is good. my best guess is there is a short connection between one of the blinker wires and the others and id guess that is
in one of the connectors coming out of the cab or the harness itself has rubbed up against something. the wiring in these trucks is pretty simple. also, if someone has added a trailer light connector might be another place to look. id suggest finding and tracing all the
wiring harnesses. just find them and visually check everything over. then get back with us with what you find