Where's the best place to find power for a center brake lamp?

[Aracer]

Is there a wire I can tap into to power, an add on LED bar lamp? Under dash, would be the easiest so I can switch it off.

[burdar]

You can tap into anything that has power all the time. Find a wire in the fuse panel or you can run a wire right off of the amp gauge. If I remember correctly, there are a couple molded rubber three holed connectors on the main dash harness. A yellow and an orange. The orange only has power when the lights are on. The yellow one might have power all the time.

[Aracer]

Sorry I wasn't clear. I'm looking for a wire that will light only when the brakes are applied. Possibly a wire that comes from the bulkhead or the flashers? Closer to the e-brake the better, since I mounted a switch in the under dash hole.
The brake light switch won't work. I think its job is grounding for the brake lamps. I have the old manual here but I can't figure out the old style of traces.

[Chryco Psycho]

Brake light switch is your only choice , should be the white wire  , it has power constantly from the fuse block & power up to the turn signal switch , after the signal switch will not work as it will flash with either turn signal

[70chall440]

Id tap into the brake wires; since you didn't say where you put the 3rd brake light I can't recommend where to tap into them but they run down the left side of the car. You have to remove the back seat and the left side rear interior panel and they are right there.

[Aracer]

Brake light switch is your only choice , should be the white wire  , it has power constantly from the fuse block & power up to the turn signal switch , after the signal switch will not work as it will flash with either turn signal

Here's what I did: I used a clamp on to tap the white wire on the brake switch. Run a white wire to a switch mounted under the dash. Ran a white wire to the rear package shelf (deck) the LED universal red and a ground to chassis. It dimmed the interior lamp and no brake lights . Obviously a short. I found 12v. at the switches white wire when brake was pressed, it must have been the grounding current through the meter. I guessed wrong.
       It seems as if the tail lights are always energized and all lighting is grounded through a switch or signal blinkers. I was thinking of running a wire from the tail lamp positive to the 3rd lamp and and ground to my white wire so it gets a ground when the brake is applied.      Which color wire do I tap in the trunk/ lamp?
    I know this is a long winded explanation. Maybe I need a course in Mopar backward energy apps.? I going out to check the tail lights for current now. And the headliner is out so I can get to the trunk easy. Sorry, but it's my 1968 cuda not the 74, but in theory it shouldn't mater. Since I'll add one to the 1974 next.

[Chryco Psycho]

Well you should have a pink wire from the fuse block to the switch when the pedal is pressed power will connect tot he white wire & go up the column to the turn signal switch where the power is divided into left & right so it can be interupted with the turn signal . so connecting to the white wire & the brake light switch near the brake pedal is the only place where the 3rd light can get power with the pedal depressed & not connect to the turn signal

[Bullitt-]

someone added to a '64 ...might help  http://www.earlycuda.org/tech/thirdlight.htm

[Aracer]

Well you should have a pink wire from the fuse block to the switch when the pedal is pressed power will connect tot he white wire & go up the column to the turn signal switch where the power is divided into left & right so it can be interupted with the turn signal . so connecting to the white wire & the brake light switch near the brake pedal is the only place where the 3rd light can get power with the pedal depressed & not connect to the turn signal

The switch must short it out when on; made the brakes not work and dimmed the ceiling, so the idea was correct.
     You know once I looked up the wiring in my 1974 manual. I could see the layout. Since it was a 1868, I tried to read that old confusing diagram and doubted myself. The test with a 9volt battery shows the LED bar still works.
     I'll get a new switch tomorrow.
Thanks for all the help everyone. 

[Chryco Psycho]

Maybe the Led doesn't have enough resistance to ground & it sees it as a short , you may have to add a resister inline

[Aracer]

The led bar was cracked in shipping so the seller refunded $5. It works testing with a 9Volt transistor battery. They have resistors for each pair of led. 
    The problem was with the switch, once I taped the leads together without the switch the light bar worked great. That damed switch tested good, too, and the leads were not touching the dash.
   Cost me $2.50 for 48 CHINA LED.  It is taped to the lower deck window, the wire runs under the rocker cover. Simple safety for the city.
     I think I'll buy the nice billet one that hangs from the upper window frame, for my E-body.
http://www.ebay.com/itm/Street-Rod-Billet-Aluminum-LED-Back-Window-Third-3rd-Brake-Light-BRIGHT-LEDs-/162107139905?hash=item25be56a741:g:MyoAAOSwzJ5XY1np&vxp=mtr

[burdar]

I wouldn't want the third brake light flashing with the turn signals.  I'd run a power wire to it and control the function with a switch at the brake pedal.  The pedal switch can control the ground.

[duodec]

Back when the high mounted third brake lights were new on the market and a popular aftermarket accessory for older cars, you could by them with a "logic module" for cars that used the same lights for brake as for turn signals (all red tail lights, and no separate brake/turn).  The would hook into both the right and left tail-light harnesses with crimp on wire taps, and only energize the high mounted brake light when both of the tail lights were on at the same time.  Turn signals would not light the high brake light but braking _and_ emergency flashers would light it.

You can still find the logic modules (probably all chinese these days) or may even make your own if your search for "third brake light logic".  Amazon and other vendors likely have them.

If you get one though please use proper soldered-in connections/taps into the existing wiring, not those awful crimp taps.

[ted]

geeze simply use the wire after the brake light switch on the petal

[70chall440]

geeze simply use the wire after the brake light switch on the petal