check out
http://www.strobesnmore.com/ , they have everything on the market related to this kind of stuff, thats where i get all i need.
I know what you are looking for, and i looked around but couldnt find one. the closest thing would be
Flashback Taillight Flasher. it might not be good for you because its a hard wire product and it will do it all the time, but i guess you can put a switch on it. this is what it does:
The Flashback alternates the brake lights against the back-up lights at 2.4 flashes per seconds. It also has an additional output for deck lights.
Again, i know its not what you are looking for but check it out, it will make you visible no doubt.
oh a buddy just told me to check out http://www.lshlights.net/ and http://www.galls.com/ and http://www.chiefsupply.com/Vehicle_Equipment .
If none of these guys have it you probly wont find it anywhere.
Im betting its not that difficult to make, if you know someone with electronic/electrical experiene, maybe they can make some kinda flasher for you.
If you find please post, im interested in that too!
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just found this
http://www.signaldynamics.com/products/Modules/xp.asp . Its made for motorcycles, but it says something about cars too. I think it can be fitted on a car also!