LEDs vs Halogen ulbs
 

Is there any particular reason that a Hologen bulb cannot replace an LED bulb. This would be for the Fog bulbs

The fog lights on the 986s ARE halogen. Now if you actually mean replacing a halogen with an appropriate LED replacement, there is no issue.

If you are talking about replacing an LED with a halogen in an application designed for an LED, which is not applicable to a 986 as I mentioned above, then yes, there is an issue. If a the application was designed for an LED, it will not be able to power a halogen bulb. LEDs use much less current and have control circuits designed to limit the current to the LED and the enclosure will not be designed for the heat of a halogen bulb. LEDs in lamp assemblies _designed for LED_ are generally not user replaceable as their MTBF (mean time between failures) is high enough that it is basically a lifetime "bulb".

A little more information would be good. What are you trying to do?