Aftermarket brake light not working. Need help

My stock brake light works, but when I changed it to a brand new aftermarket brake light it only works properly when the ignition is in the second slot (park I think?). When the ignition is turned once, the brake light turns on but it only lights up very slightly when the brake is pressed. However when the ignition is turned to the furthest position it works normal. I am in desparate need of help, and yes I have seen the wiring diagram a million times.

I don't have a quick and easy answer but if it were me, this is how I would go about it.
Do you still have the stock taillight?
What's different between the new and the old?
Is one LED and one incandescent?
Do you have a meter?
If they are both incandescent, then use your meter and make sure they are wired the same. If they aren't, figure out why not, and correct it. Verify the bulbs are the same or similar.
If they are wired the same, I would suspect my switch. Check the voltage to the lamp in both ignition switch positions.

Most Kawasaki brake lights are rubber mounted with three wires for the two circuits:

BRAKE/TAIL LIGHT Can be an # 1157 dual filament bulb
RED, Running or Tail Light.
BLUE,(sometimes with a Red stripe) Brake Light Circuit.
BLACK with YELLOW stripe, the ground circuit.

If your new brake light needs a good ground to work, you'll need to make a ground wire, or as mentioned, the ignition switch may be failing. Get a good multimeter and check continuity and electrical voltages. NEVER check continuity(OHMS settings) with voltage on the circuits, as the meter will be damaged!!

The bulb works, because the light works properly when the ignition is in in the park position. My voltages at the brake light wires are 9.10V when the ignition is in the "on" position, and 11.05V when in the "park" position. Can a 2 Volt difference really keep the light from working? And if so, does this mean that the ignition switch is bad? I would like to know readings from other bikes.

1slowkayzee wrote: ... and yes I have seen the wiring diagram a million times.

Would trace the wiring diagram and measure voltage at each juncture, to determine where voltage is being lost.

Might be inside the ignition switch. Sometimes cleaning a connection and spraying with a dedicated electrical cleaner such as Deoxit might achieve better conductivity.

Good Fortune!.

When the battery reads 11.3V the brake light wires read 10.4V. Is this normal? As soon as I plug in the oem light it works fine.

Is the new brake light LED or the kind with a bulb?

1slowkayzee wrote: When the battery reads 11.3V the brake light wires read 10.4V. Is this normal? As soon as I plug in the oem light it works fine.

Ideally, any positive reading at any location within the wiring harness should match the battery voltage as measured between the battery posts.

Would suspect some corrosion in one or more of the connectors and/or switches in the circuitry between the battery positive post and at whatever point the comparative voltage is measured.

If not already done, would also assure integrity of the ground where the negative battery cable attaches to rear of engine.

At 11.3V, the battery likely needs attention or replacing.
Would remove it, assure fluids are at the correct levels, trickle charge overnight, and have it load-tested before using it again.

Good Fortune!

Difference in voltage from one position to the other? Yes, that much drop can cause your bulb to dim like you have seen.

Could be the contacts inside the switch are dirty. Can't (or not supposed to) open the ignition switch to clean it. The other thing is your headlight on/off switch. I'm guessing you have one on a '75. Those are out on the handlebar end and get hammered by the elements over time. That one you can clean. Follow Patton's advice and trace the circuit back and look for where the voltage diminishes. Verify your battery condition, too!

What is odd is that your stock lamp works. I would still make sure the bulbs are the same between the two units. Can you swap them with each other? If your new one needs more poop to work properly it's probably going to be a lot brighter once you get it working, which is a good thing, but it has also uncovered an issue with your electrics that you now have to correct, which is also kind of a good thing.

The aftermarket brake light works perfectly fine,but only when the ignition switch is in the park position. Even with the voltage deviation, the oem light is very bright when the ignition is in the normal position. Also, the voltage is low because my charging system doesn't work.

Yeah, that's the head scratcher.
A quick look at the wiring diagram shows that when your ignition switch is in the PARK position, you are getting a direct feed from the battery to the rear lamp. When you are in the ON position, you are sharing your feed with the dimmer switch (no headlight on/off switch, my bad) and just about everything else on the bike. Gotta love these old bike electrics!
It's looking like your new taillight is needing more current than you are supplying. Tell me, is this problem still there when the bike is running? Have you checked the voltage at the taillight while the bike is running? If it is still low, you may have charging problems, or a battery that is so flat it's dragging down everything. Verify your charging system. If that is good, then you got "ancient wiring harness syndrome." You will have to go through and clean all of your connections, make sure your fuse box is clean and tight. The list goes on and on. Me personally, if I wasn't in a position to go through every little thing on my harness, I would make sure my charging system and battery were sound and then I would just look for a bulb that was compatible with my taillight that works with the current I can provide to it.
:S

So to clarify, my problem is that my oem bulb requires less power than the aftermarket one, and that is why the aftermarket one only works when the ignition is in the park position; because of old rotted wires not supply proper power? Sounds like a good theory, I will start replacing wiring and checking continuity/ voltage readings as I go. Eventually I want all new wiring though.