I’ve been looking for a bi-color white and red #1157 bulb for just the same purpose… fitting a brake light to the back-up light in my Mark IX. All I’ve been able to find, at least here in the colonies, is an 1157 bulb from Super Bright LEDs. When you look at the specs, however, note that the red LEDs project from the end of the bulb, but the white LEDs project from the side of the bulb. This is not suitable for the purpose; is anyone aware of a U. S. source for the bulb that is supplied by Classic Car?
Thanks,
Jay
Well, I finally got around to setting up the third brake light using a two colour bulb in the back up light location. I have a positive ground and finally located a compatible bulb. I wired it in to the right hand brake line and it works really well. Now I have three very visible brake light.
However and now here’s the kicker I have done something wrong because now when I put the left turn signal on bothe the turn signal and the center light flash. This only happens with the left turn; the right turn works fine.
Two questions:
a) does anyone have an idea what I might have wired incorrectly and;
b) Does anyone have a copy of that DLOC magazine article from March 2021 that they could share. I haven’t been able to find a copy.
thanks
Rob
Have you added LED indicators ?
They normally require a different flasher relay
If you have not added LED indicator bulbs, it may just be a matter of disconnecting the new wiring one connector at a time till you discover the issue
I will say I have a similar very weird indicator ghost in my 420G.
very occasionally over the years, it has done a similar thing
It must be somewhere in the 54yr old wiring
I appear to have solved the problem by selecting a definitely known good indicator signal, and using that to control the other, and adding a separate earths for the suspects
Hi Tony,
Thanks for the response. Yes I have added the correct LED relay.
Ah yes … 63 yrs old wiring. I’m dicovering the joys of this in so many places 
I suspect you are correct and I’ll just have to go back and check each connection separately.
cheers
R
the flasher relays can be suspect
I personally decided LED indicators are a waste of time
The other one not to replace is the alternator light (if fitted), if one does, a resistor must be added
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I have successfully (almost) set up the third brake light in with the center backup light.
But this center light keeps coming on when I use the right indicator. I’ve checked all the possibilities in the wiring in the back of the car (to try and resolve this) and I think the only solution is to go further upstream and connect into the green/purple from the stop lamp to the flasher switch.
Can anyone tell me where the connector is located on the car? (see attached diagram)
FYI: This is a Mk1 and not a MK2 and after 63 years all the fabric covered wires look brown now.
cheers,
Rob
Jaguar did some convoluted designs in order for the rear lights to use two filaments to do three jobs. In the XK140, in order to use the single filament to light both with the brakes and the turn signals, Jaguar used a complex relay that disconnected the brake light on the side when the turn signal was activated, allowing that filament to flash. In the MK1, that function was incorporated into the turn signal switch. That means that the power to the rear lights will be on when the brakes are on AND flashing when signaling a turn. There is no way you can use the existing wires to activate your third light, since both rear lights are doing double-duty. Your only option is to tap into the power from the brake switch before it gets to the turn signal switch. The easiest way to do that would be to add a wire to the switch itself, which is located inside the right front fender near the booster, IIRC. Since it is outside the car it might be easier to run it to the back of the car. The connector in your diagram is inside the car, next to the steering column.
Well that did the trick. Thanks again to all who shared advice.
I ran the new 3rd brake light wire up under the parcel shelf. Ran it down the side of the back seat and then underneath the carpet and the door sills on the (North American) passenger side then up under the dash and over to that bundle of connectors (shown in your picture). Happily I was able to identify the Green/Purple brake light wire (the plastic colours were exposed). Then I simply undid the two existing bullet connectors, put them into a double connector and added the new line from the 3rd light.
Voila, all the connections for the brakes are now upstream of the turn signal switch and they work like acharm. No more flashing the brakes when the turn signals are activated.
Very happy with the results and with the knowledge that, now the [LED] brakes can be seen even in the sun.