Here it is: I had only one fog light on my 88' when I bought it. Whenever I turned the foglights on, my headlights would lose power and my foglight would barely light my way. I installed the missing foglight and changed the alternator because it was almost dead. Also, my lights would dim once a while but only when my foglights were on. After installation was done, with everything installed correctly, the problem didn't go away. My headlights still lose half of their power whenever I turn my foglights on and the foglights barely barely offer any lightning. I checked my fuses and everything is allright. What could it be? Thanks a lot!

 

Loose wires, bad ground? I would get a wiring chart and see whats been spliced into that shouldn't of been.

 

https://mustangforums.com/m_616330/m.../tm.htm#616414

 

check for a bad ground, then follow the wires and see if there are any breaks in them. could also be teh switch