GM-GMC: no lights, gm trucks, auto parts stores

Question
I have a 1987 GMC truck whaen i turn my head lights on i have no brake lights or tail lights my turn singals are weak . When my head lights are off my  breaklights are working and my turn singals are not weak. my hazard lights work just fine with my headlights on. any answer you have will be helpful tahnks.  

Answer
This is likely to be a tough problem to solve.
I would try a new headlight switch.  My 1983 and 1984 GM trucks required new headlight switches not long after I purchased them.  The switch can be defective internally and cause a lot of problems like you are experiencing.  The switch  is readily available at most auto parts stores and is usually less than $15.00.
If that does not help, you may have a problem inside the wiring harness and that will be hard to find.  If you can find a shop manual, you can determine the color codes for the wires running to the lights and trace them all down.  Older vehicles often wear the insulation off their wires or the insulation becomes brittle from old age.  This can cause circuits to short out or cross-connect and make lights and other accessories do strange things.