I have a 2002 ford f150 , I had her parked for a while and when I got her going, I had no power to the taillights. Brake, turn signals, Hazzards work. But when I turn on the light switch I get power to the headlights but no power to the taillights or the side markers. Also when I turn off the headlight switch they stay on. And info I what it can be causing this issue

 

So your headlights low beam stay on after you turn them off, do the marker lights also stay on?
Your rear parking/marker lights do not work but the rest of the rear works, correct? And youve already checked for voltage and found none at the bulb.
What are the side markers you mention, the marker lights in the headlights maybe?

 

Yeah low beam stay on, yes the side markers on the headlights, i had to bypass a wire to get power to the side markers and taillights
also light bulbs are good

 

For the taillights everything works on both sides except the marker lights on both sides, correct?

 

The side markers don't turn on or the rear taillights , but the headlights turn on. When I turn the switch off the Headlights stay on . I checked for power to the rear taillights but I'm not getting power to them

 

Test fuse 6 in the battery junction box, check for voltage at both sides.
Next remove the headlight switch, check for 12V at pin 10 of the headlight switch connector.
Connect a fused jump wire from the positive battery terminal to pin 12 of the headlight switch connector, see if the marker lights come on.