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Third Tail or Additional Brake Light ( ST1100 ) *

Started by KoTAOW, March 28, 2009, 01:07:00 PM

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Original "How To" article written by Ted Norris, March 1998.


This installation makes a VERY significant change in the brightness of the tail light, set aside about 3 hours to do it.

    You will need the following:
    1. A 55 watt H3 bulb (Std. driving light bulb)
    2. About a dozen(12) male and female spade lugs
    3. 5 ft. of red and black wire
    4. A fuse holder and fuse (I use the flat type, same as the ST has)
    5. A std. horn/light relay (their usually rated at 30 amps, this is way more than
       you need but there cheap and can be found everywhere)
    6. A 1/2"(.500) drill bit and drill( I recommend a sheet metal drill, a std. drill bit
       will work though.
    7. A small burr or rotary file
    8. A soldering gun and solder
    9. A pair of "small" sharp side cutters
   10. Blue Hi Temp Silicon

    OK, First take off the seat, grab rails, trunk assy if you have
    one, saddle bags, left side panel and the fender.
    Pull the tool box flip door forward and unplug the light assy.
    Unbolt the rear section, now spread the sides out far enough to
    clear and lift, there are 4 posts that the grab rails bolted to that must be
    cleared, and slide it back.

    Take the 4 screws out(2 inside/2 outside) and rotate the brake
    light assy out from the inside.
    Separate the wiring and bulbs from the housing, DO NOT TRY TO
    SEPARATE THE LENS FROM THE HOUSING, it s a sealed unit and very expensive.

    Right where you want to put the light there is a X like
    protrusion, use the  small side cutters and cut it off as much as you can, use the burr
    or rotary file to smooth thing out.

    At the center of the X is where you want to drill the 1/2" hole.

    If you have access to a drill press I suggest you use it, if not,
    be VERY CAREFUL, when drilling the hole, the plastic is very soft
    and the drill could screw it s self, when braking through, into the lens,,,you don't want to
    do that. If you had everything lined up right the hole will be centered on the
    protruding part of the reflector. Blow the debris out and set it in a safe place.

    Install the wires to the battery. and ground, leave the fuse out for
    now. Follow the instructions for using the relay that are on the box or info
    sheet that come with it.  Use the GREEN wire with the YELLOW stripe as the power
    source to  trigger the relay.

    Time to light up the solder gun, solder a pigtail on the base of
    the light for  a ground, get the lens assy and push the bulb into the hole, make
    sure that the light base is straight up and down(I), you will notice that there
    are gaps top and bottom because of the shape of the reflector, place some tape
    over the gaps,  use the Blue silicon to make a gasket that will keep water and
    who knows what  else out, just keep in mind that you may want to take the bulb
    out someday.

    Mount the relay to the lower part of the flip door right where it
    changes angle.  You may find some where else but this worked for me, I used velcro
    to hold it in place.

    Reassemble the back end, hook things up, put the fuse in and try
    it out. Looks great don't it?

    Put everything back together and go show your friends.

    If for some reason you want it to flash just put a std turn signal
    flasher in line on the ground side.

    Hope your as happy with this mod as I am.



Additional information and pictures provided when I did the above mod to my ST.
I used the additional light as a flashing brake light.

NOTE:  Additional pictures are no longer available.

Parts used:
55 watt, H3 bulb at $4.99
Heavy Duty auxiliary relay, 30 amp, 4 terminal, Nightblaster number A-715 at $3.95.
Heavy Duty flasher, 12 volt, 2 terminal, Littlefuse number FLR 536/552 at $2.99

Schematic ( PDF ) can be found below.

Picture #1
This is a view of the tail light assembly before any modifications.  Notice the X shaped piece of plastic extending from the center.  This is the exact location of where the high intensity bulb will be located.  This plastic must be cut away with a pair of side cutters and a dremel tool.

Picture #2
This is a view of the assembly after the X shaped plastic has been removed, and the 1/2 inch hole drilled in the exact center.

Picture #3
H3, 55 watt halogen bulb with ground wire attached using a soldering iron.

Picture #4
Tail light assembly shown with H3 bulb inserted in 1/2 inch hole.  Bulb gets RTVed into this location and wires secured with a tie wrap.

Submitted by Tom Melnik  STOC #346


Submitted by Norm Keller, STOC#8030


Third Brake Light ( ST1100 )

I added an H1 quartz bulb linked through a relay and load type flasher to the rear brake light wire.

It is a bit brighter than the other two (side) original brake lights but not enough to draw "official" disapproval. When the brake is applied, the three bulbs light and after about two seconds, the load type signal flasher begins to cycle which creates a very eye catching effect. I used an old KLR650 signal flasher because I wanted a delay before the flashing begins and an electronic or heavier load type begins to flash immediately.

Easy to do and, IMO, well worth the time required.

I seem not to be able to find photos of the bulb install but someone else posted these so they will be found.

Identify the brake light wire in the rear plug behind the under seat storage panel.

Remove the brake light terminal from the junction plug. Note that I do not like to cut into wiring and prefer to add the lead to the original plug in such a way that the system can be returned to stock if desired. The special tool is used to push the little latch lever in to allow the terminal to be withdrawn from the body of the plug. Bend the lever out again before sliding the terminal back into place.

Solder the lead wire to the terminal, positioned to allow the terminal to be replaced into the plug body. The other end of the lead wire has a female bullet connector to allow the third bulb harness to be connected or removed for service. By the way; if you are in the market for a solder gun for shop use, get one of these Wall LG400C units. Mac Tools sell them as do other sources. They are so superior to the transformer type made by Weller and such as to beggar comparison. The weight is a fraction and much more compact. 400 Watt output heats up in a few seconds and has enough go for heavy terminals. If that's not enough, get the 550 Watt element. I've seen them advertised for around $60 yes, I own one, and yes, I used to sell them. The other types are crap! As any alternator repair shop!

Power for the relay was taken from the fuse box and is live because the circuit is controlled by the brake light switch.

The terminals of the 10 amp miniature relay were potted with a rubber tool handle coating after the wires were soldered into place. The relay was secured to the left side, outside of the rear seat storage compartment so no storage space is lost.

A search under 3rd brake light or such should find the photos showing the modification of the tail light housing. A very simple and useful modification, IMO.

Hats off to the originator!


Thank You again for your contribution Norm Keller, STOC #8030