Friday, April 25, 2014

Getting My Signals Crossed

This blog entry started way back in March with VMF user "Robsan" posting a request for anybody whom has been able to rewire the 68 Mustang's front marker lights to work as one might expect them to.  Before this question makes any sense to those of us that either don't own a 68 or simply haven't noticed the eccentricity of the front marker/signal lights let me give a refresher course on the factory function of these lights.

When the headlights are off, the front side markers are off and the front markers under the bumper act as turn signals.  When the headlights are switched to the parking position but with headlights off, the side markers come on steady but do not act as turn signals whereas the front markers under the bumper turn on stead AND act as turn signals.  Finally, when you turn on the headlights, the unexpected occurs; the front markers under the bumpers turn OFF but still act as signals but the side markers remain lit and still serve no signal function.

Long story, short, Robsan developed a wiring harness that is installed between the factory engine compartment light harness and the front marker lights that would supposedly solve the issue.  He had gone on to post a link to a couple of videos where he demonstrated the functionality and sure enough, the harness allowed both front marker lights to act as signals AND markers that were constantly on while the headlights were on... exactly as one might expect them to function.

This appeared to be well thought-out and completely reversible modification so I requested more information at which point he made me a customized harness which included a tap for my halos to allow them to remain on solid while allowing the front markers to act as signals.  He posted a video for me to demonstrate the new harness and it's functionality and I was sold.  I purchased a harness set with which he also included a pair (3 actually) dual filament, amber bulbs modified to fit the Mustang side marker socket.

The installation would normally, literally, be "plug and play" but due to my halo headlights, I also needed to modify my halo power leads to utilize the new connector that he provided to tap into the adapter harness.  This was a matter of cutting the connector off of the halo power lead and de-soldering and repairing the side marker lead where I had previously tapped power for the halos.  To provide power to my halos I simply soldered the connector that was provided with the harness and plugged it into the provided lead of the adapter.

The installation of the adapter itself, sans the halos, would have been to

1) Unplug the front and side marker leads from their factory supply connectors.
2) Plug the harness input connectors to the factory supply connectors as marked on the harness
3) Plug the side marker lead into the appropriately marked output lead of the adapter
4) Plug the front marker lead into the appropriately marked output lead of the adapter
5) Screw the the ground lead of the adapter to a ground point on the chassis (my side marker housing)
6) Replace the side marker single-filament bulb with the provided dual filament bulb.

Then button everything up and use the headlights to confirm that everything does indeed work as advertised, and it does.


Side and front marker leads removed from their supply connectors and replaced with the harness input leads.

Adapter ground lead attached.

Side and front marker leads plugged into the harness outputs

The new dual filament bulb (in hand) and the soon-to-be-replaced single filament bulb.

Dual filament bulb in socket

Wires buttoned-up.

Success.  Not really possible to show the signals in operation but both the side and front function while headlight is on or off and both lights also come on when headlight is on.
So that's all there is to it.  I recommend that you shoot Robsan a PM if you want this mod on your own 68.  He does some top-quality work.


  1. I guess I never noticed or thought about how the turn signal and side marker lights would work on older cars. I think my 68 Camaro was the same as the Mustang, except the parking lights did stay on with the headlights but the side markers didn't blink with the turn signal. Thanks for the info and Robsan's info.

  2. Nice job Alex. Since I have a 67 this shouldn't be an issue on my car, but it is good to know nonetheless.