I notice this thread was started almost 5 months ago. So if you're still thinking about it Nate, and you have a hard tonneau cover, you might want to consider what I did.
I removed the 3rd brake for the top of the cab (I don't think most drivers notice it up there anyway) and put it in the spoiler.
That puts the light at about eye level of the driver in the car behind you.
If you don't have a cover or you don't want to go the spoiler route, and still want to go with the blinker, check the motorcycle mags or a cycle shop in your area.
I have a blinker on my GoldWing. It makes all 3 (2 regular plus 1 in the spoiler) brake lights blink rapidly. I'm sure the literature that came with it said they also worked on cars.
But I've had the blinker for over 10 years now.