Headlight wiring issue on 87 GT

I bought the car used with some issues. The tach was reading way off (really high) and when I turned the headlights on all the gauges went buggy.

I ordered a new instrument cluster (140 mph) out of an 89. A new headlamp switch with connector and pigtail, and a new dimmer switch.

I tore out the instrument cluster and dash to check the wiring. The plug to the Multifunction switch had been replaced by a mechanic (the previous owner told me about it. He said he had paid over a thousand bucks to have a mechanic trace down a short). The replacement had white wires that were crimped into the existing harness.

The headlight switch wiring was correct (I lucked out and found a KLM 87 Mustang wiring diagram manual) with the exception that there was a light blue/black striped wire spliced into the red/yellow striped wire that goes to the multifunction switch. I checked the wiring diagrams and realized this was the power lead coming from the fuse block that was supposed to go to the fog lights (which are no longer installed). When I spliced in the new headlamp switch connector I omitted the LB/BK wire. I installed the new dimmer switch and instrument cluster and connected the battery back. At this point everything worked fine except the low beams. Flash to pass, high beams, turn signals, guages are all fine but no low beam headlights.

So I disconnected the battery and spliced the LB/BK wire back into the R/Y wire and voila..headlights and all is working well.

I have no idea why this is working as the power provided by the fog light wire is on the outbound part of the headlamp switch and in theory should not be affected by the switching. In other words this circuit should be hot without the headlight switch.

I disconnected the LB/BK wire from the R/Y wire as I want the car wired correctly to avoid any problems down the road.

Any suggestions as to why someone would wire it this way? Why is this working? Any help would greatly be appreciated.

Also, Does anyone have a source where I can get the replacement connector to the MF switch? Preferably with the correct wire color and not just white wires.

I found out what was causing my issue. Turned out to be a bad dimmer switch and whoever had worked on the wiring previously replaced the connectors and wiring coming from the back of the multi-function switch and then just bypassed the normal (non high beams/flash to pass) circuit using the LB/BK wire for the foglights to close the loop. So basically they just powered the headlights by piggybacking the R/Y wire coming from the headlight switch to the dimmer switch and ran it to the fuse block then the headlights. This just goes to show you what goes on when you take your car to a bad mechanic. This wire was half the size as the normal one and would surely cause some problems down the road. Not to mention the fact they were powering a rail on the fuseblock that had other items on it not related to the headlights. I took the MF switch apart and sure enough two of the brushes/contacts on it were burnt out so I shelled out $136 for a new one (gotta love ebay) and will be putting the wiring back to stock while i wait for the part to arrive.

This was indeed a learning experience but damn I hate fixing things that I didnt foresee would need fixing when i bought the car. Next stop...checking the fuel level sending unit and replacing the temp sensor.