ok here we go...i have a 91 fc just so you know. i have some sort of water leak where i end up with a small wet patch on my driver side floor when it rains and let me tell you i have been searching for the leak for months with no luck. here is the real problem- the other day my headlights would not turn on so i drove to my parts store and replaced the fuse....about 4 hours later i turned my headlights on and smoke started seeping out of the seams in my dash. i pull over and flip out while i rip my dash apart only to find my headlight harness and clip to be all melted......WHAT DO I DO....WHERE DO I START.....WHERE IS THE PROBLEM COMING FROM.......im pretty sure the water leak is a big part of it but i have never seen water near my fuse box...im great with this car but im horrible with its electronics.. someone just please tell me where to start with this issue

 

If you have a sunroof, the drip tray under the cap has a drain tube at each corner. If bugs or debris block the drain, the water comes in.

Sometimes the dash harness damage can be repaired.
If not, it can be replaced seperately from the main harness.

There is a link to PDF's of the factory service manuals in my sig below

 

The harness to the headlight switch is a common problem on these cars. A combination of not-so-good solders from the factory, and a little too much current through the wires gang up to melt the harness. You'll also find out about the wiper switches and the clock module on FC's, too. And bad grounds, too. And the short-start flooding problem. And the 3800 rpm hesitation, too. And etc.......