LED taillight (kz400)

I'm sure you're right about all that, but also it seems like the bike may have been missing the switch when it came to me. Per the manual I think it's supposed to be here. Also finished wiring the rear blinkers and unlike when I just had the right side on and they were working as expected...now I get all four going when I hit the signal no matter which direction....

You are looking in the wrong place at the wrong thing, the below picture you posted has the answer for you !!!!!!!
Ref the turn signals, you have a single warning light so will need to install "diodes" in the wires to the warning light (Scirocco has previously posted a diagram for this)

 

Thanks and so I should add the resistor to the circuit? They came with tap connectors and I'm a bit leery of using of as I'm worried they'll ruin the wiring and doing some searching it seems they may not be a great solution. Also I've no idea what's in that photo, I pulled it from the manual posted online and can't make out where on the bike it is or what is in the pic that's helpful.

Also what's missing that's meant to plug into the brake fluid reservoir? 

No resistors, I will try later and sort a diagram for the "diodes" for the turn signal warning

Does the brake light work when you pull the handlebar lever? (If so nothing is missing)

Thanks, cause I'm not understanding otherwise. Also it seems this likely needed something plugged into it but can't find anything in the manual.

Does the brake light work from the lever ? (that is the brake light switch and should have 2 wires from the switch loom to plug into it unless it has a pressure switch in the brake line)

Believe so but will have to confirm later. Can see the brown and blue wires that I believe are meant to go to it go under the backside of the tank but can't see where they end up and didn't have time to pull the tank last night.

Here is the link for a conversion kit ref the turn signal issue

Metric Bike Diode Kit | Super Bright LEDs

I am about 83% sure that all of the disc brake equippped KZ400s, except the 1979 KZ400H, had a pressure switch on the splitter. The 1979 KZ400H and the later KZ440s had the pressure switch on the master cylinder. 

Your signature line says yours is a 1976. Since it has a front disc brake, it would be a KZ400D, which should have a master cylinder with a round reservoir, and a pressure switch on the splitter. 

www.partzilla.com/catalog/kawasaki/motor...cylinder-76-77-kz400

Evidently, yours has a later model master cylinder from a KZ400H, or a KZ440, with what is now a redundant pressure switch.
 

Thanks and seems that will probably solve the blinker issue but why didn't it happen when I swapped the front set? Or when I had one of the two rears installed?

Believe you're right that it's a kz400D, as it's got the disc brake and electric start. Bike definitely wasn't 100% original when I got it but pretty close so maybe that was already swapped? Is that something I need to replace to fix this problem?

While there was still an incandescent bulb in either side the warning light had a path to ground through the bulb on the opposite side. Because an LED will illuminate on a smaller voltage that path to ground also illuminates the opposing lamp