I have a 99 chevy pu and the headlights r not working. i had a mechanic look at it and made it where the bright lights would only work because he said the multy function switch was out but now no lights work so i replaced the headlight switch and the multy fuction switch and still no head lights i do tho have running lights and turn signals

Maybe headlamp relay . Do you know what a relay is ? How to test a automotive electrical circuit using a wiring diagram an a DVOM - digital volt ohmmeter ?
Free wiring diagrams here http://www.bbbind.com/free_tsb.html
Enter vehicle info Year , make , model an engine size. Under system click on lighting ,then under subsystem click on headlamps . Click the search button then the first blue link . Looking at the diagram you will see the headlamp an panel dimmer switch ,on top of that you see the fuse that powers the headlamps ? Test it with voltmeter ,is it hot ,does it show B+ voltage ? Go to the headlamp relay ,pull it out . Look on the bottom of the relay , like at the wiring you will see four set's of numbers . 30 & 87 - 85 & 86 . Turn the headlamps on an test for B+ voltage at terminal 85 , where the relay plug's into the fuse box. Test for B+ voltage terminal 30 , again where the relay plug's into the fuse box. Volt meter on ohm's scale 87 & 86 should have continuity . If you have all that ,test relay . Videos on youtube showing how .
Relays Understanding Diagnosis and Testing Relayed Circuits Pt1

VEHICLE RELAYS Operation Diagnosis

Check/ replace the light control module