Seperate BRAKE & Turn signal lights...

It is a 3 prong plug from the wiring harness to the light correct?

One will be ground
One will be driving light and brake light
The last should be the signal.

CRAP! Forget that ^ it's wrong. There is ground, parking/driving, and turn/brake. Many of those are run that way right from the dash area. You could wire in a unit for a car that is used to convert from that setup to a 7 or 5 pin setup for towing. No reason why it wouldn't work for our purposes as well.... Let me try to find something on the net of what I mean and I'll post back in a few.

 

Found them.....

http://www.ontheball.com/taillight_convert.htm


Converts from 2 or 3 wire taillights to separate lights. You could use it to setup tail lights on all 4, brakes on 2 inner, then turn on 2 outer.

 

go to the 7 way and you should find seperate brake light and turn signal as that is the way the trailers have to be wired

 

What you want to do is run a separate 7 wire to the tail lights, you can just piggy back off the trailer wiring and rewire to how ever you wish. I had a customer that had a sequence set up that he had on a 8 light tail light set up so they blinked like an old FORD.

 

On the firewall under the hood, drivers side down low there is a plastic cover that hides the various harnesses that come out of the cab. You could do some creative rewiring there with the trailer harness and the tractor taillight harness. At least that way you won't be running new wires all the way from one end of the truck to the other. In fact, if you used the wire that normally operated the back up light and hooked it to your new extra brake lights you could probably do it fairly easily and not really hack up the wiring too much. The down side is no more back up light.

 

It's been a long time since I worked on a problem like this, but I think the wireing that causes the brake lite and the stop lite to use the same filiment is in the turn signal switch. On a 378, on the steering column.
If you can check somehow and see if there is a wire that is hot when the brake lites are on that should tell you. ( no other reason for brake circuit to be with turn curcuit).