I always wondered why my coach came with just one side turn signal. I have a marker light mid way down the body but it's just a marker, no signal. I got replacement lights that are a combination marker/signal but, even though they dimensions were correct, they didn't fit the recess in the fender skirt. My next idea was to find a 3 wire socket. That didn't work either because the marker uses a 194 bulb and there is nothing equivalent to it in dual filament. I almost gave up until I found this webpage. The text of it is below. Basically, what you do is disconnect the ground of the marker light and hook it to the positive side of the turn signal. The marker light gets it's ground through the turn signal bulb. When the turn signal is on and the marker light is on, the marker light blinks opposite to the rear signal. When the marker light is off, they flash in sync. The hardest part was getting a wire to the turn signal feed. I plan to add a signal at the back of the coach so I have 3 turn signals down the side.
Daniel Stern Lighting Consultancy and Supply
Vehicles with 2-wire, bulb-type side markers have come from the factory two ways: with the front side marker bulbs wired across parking lamp feed and turn signal feed, which provides a flashing side marker, or wired across parking lamp feed and ground, which results in a non-flashing side marker. There's no rhyme, reason, or consistency to which vehicles have what; front side markers may
flash with the turn signals, but they're not required to, so it's a matter of automaker whim; the Jeep Wrangler had flashing front side markers until 2003, non-flashing starting in 2004. It's pretty random and arbitrary. If your vehicle has two-wire front side markers with filament bulbs, and they don't flash with the turn signals, you can make them do so by moving one wire.
Here's how the front parking, turn signal and side marker lights are wired so the side markers do not
This diagram shows the common "park/turn" bulbs, with a bright turn signal filament and a dim parking lamp filament in the one bulb. Each filament has its own feed terminal on the bulb base. The wiring is the same in systems that have separate bulbs for the parking lamp and the turn signal; the only difference is that the park lamp feed and the turn signal feed go to separate bulbs.
With this hookup, the side marker lamps are wired exactly like the parking lamps. They are always grounded, and they receive +12V whenever the parking lamps are on. Therefore, they illuminate steadily whenever the parking lamps are on, and never flash.
Here's how the system is wired to make the front side markers do double duty as side turn signal flashers:
There's only one difference between this setup and the non-flashing one: Instead of the side markers being wired across parking lamp feed and ground, they're wired across parking lamp feed and turn signal feed
The side marker bulb socket is isolated from ground, and one bulb lead goes to the running lamp positive circuit. The other lead is tied into the turn signal positive lead.
When the parking lights are on and the turn signal off, it grounds throught the turn signal filament and illuminates the marker lamp. When the turn signal flashes, it interrupts the ground and the marker will flash. When the running lights are off, the process is reversed, with the ground being through the filaments of the running light circuit, and the marker will flash in sync with the turn signal instead of alternately.