REDLX50 is correct, the fog lights are powered by the low beam circuit and the increased amperage overheats the wiring and switches and can actually melt the switches and cause the headlight circuit breaker to cause the headlights to flicker. I'm going to be re-wiring my fog lights soon where i'll have a new power wire throughout (i'm upgrading to 130Watt fog light bulbs) and have a relay coming off the parking light circuit for the control side and a switch to control the fog lights (needs switch and relay to be activated to turn on). It should be pretty easy to rewire and all it will take is like 14 gauge or 12 gauge wire with a fuse or circuit breaker on the fog light power circuit and the appropriate control devices. This site actually has a technical article about the upgrade that is very good and detailed. Good luck,
------------------
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT