DeanG posted a question on se7ens mailing list about wiring a combination brake/turn light into his car.
You can go buy the trailer kits that convert the 5 wire car systems (with amber turn signals) to 4 wire trailer systems. But what's the fun in that?
The cheap DIY way costs less than $1 (well, $4 if you count the gas for driving to radio shack and back).
Essentially, since the brake light does double duty as turn signal, you need to isolate the brake circuit from the turn circuit by using a diode, otherwise your turn signals will function like 4 way flasher.
Here is a diagram of what i did - you need to do it on each side.
[corrected wiring]
basically, on each side, there needs to be 2 paths to the light:
1. 12V--------- brake switch ----toggle switch------ light -------------ground
2. 12V--------flasher-------toggle switch--------light-------ground
I used two toggles to accomplish this each toggle has its center wired to the brake light. and the other 2 legs are in paths 1 and 2 above.
You will need to run the front turn signal lights separately (you need to use DPDT switches), otherwise your front turn signals will come on with your brake lights.