Ok this is positively too easy.
I had to mount some new side markers after slightly trimming my factory fender flares. I opted for some LED style so it would be small and bright. But I also wanted my side markers to blink when I turn on the turn signal. I reviewed the many places where this was done previously by others and it sure seems simple enough. Well in fact its easier than that.
First off the side markers do not light up unless you at least have the parking lamps on. This also means if you do what I'm about to show you do not have a turn signal either, unless at least your parking lamps are on. Note: There are ways around this and if this is more what you are looking for I suggest using the search feature on this forum or using a search engine such as Google. This is a simple fix and should not take more than an hour by anyone to complete.
Fortunately I have cleared away some fender so you can more easily see my adaptation.
On the factory harness you have a pig tail that runs from a front cowl mounted connector to the side marker lamp.
Here is a picture of the pig tail removed.
For my new LED side marker and turn signal I needed to snip off the factory side marker connection. You will not need to do this if you are keeping the factory side marker. So don't cut off any connectors unless you are also replacing the side marker lamp.
In this picture you will see where I exposed the black wire that is on the body side of the side marker lamp pig tail and snipped this black wire. Remember this is on the side that connects to the rest of the harness NOT where it connects to the lamp.
I then located and isolated the positive wire for the front blinker which is conveniently located next to the side marker lamp pig tail. The front lamp is using a 3 wire connection to have markers and turn signals in the same lamp. But the factory side markers are just 2 wire connectors so we need to make a slight change to make the side marker also blink. The front marker positive blinker wire is the one closest to the red tab on the factory front lamp harness. Sorry I forget the wire coloring at the moment.
At this point I simply attach the black (ground) wire from the side marker to the positive front blinker wire. I used a quick connector for now and will eventually get back to this and cut and solder this connection.
Guess, what, your done.
Put everything back into the wire looms and secure the looms with electrical tape. Now your side marker will blink when you turn on that turn signal.
The thought behind this is also simple. The side marker is a two wire connection. These two wires are 1 for positive and 1 for negative. Correctly connecting these wires to a power source illuminates the lamp. Interrupting either connection and the lamp goes out. In this rewire of the side marker we moved the ground wire from a constant ground to a ground that is grounded when the front turn indicator portion of the front lamp is off and is positive when the front turn indicator portion of the front lamp is on. This results in a steady light when the turn signal is off and you have a normal side marker. When the turn signal is turned on your ground connection will change from ground to positive in a rhythmic cycle and your side marker now blinks.
And to show you what I did, here is my new LED side marker / turn signal.
And here it is installed.
This is a really simple mod and if you want you side markers to also blink, there it is.