Hello Everyone, I have a 1973 RT3. Runs great and is still running. However, all brakes lights stopped working, turn signals, and horn. I have checked continuity on every single wire and everything is solid. I then noticed my battery was low. my trickle charger (which I keep on all the time) said the battery was toast. sure enough, it is only putting out about 4 volts. 

When I temporarily hook the charger up, the horn will work again until the charger realizes the battery is toast and then it will turn off. 

When I turn the ignition switch to position 2, the headlight comes on and so does the tail light. 

I started the bike up today, and I was getting roughly 5 volts, I would assume that I should get about 7 or 8 to charge the battery. I checked it on AC and DC because I have no clue how this system works. I also checked the rectifier with my fluke and that seems to testing good. when I hooked it up, I got 0.5 and the beep when the positive lead was connected to the white wire nd the negative lead was connected to the red wire. 

If I could get help understanding what voltage I should be getting, and what could be causing it to be low, I would greatly appreciate it. 

Things to note: original wiring harness, plugs foul every now and then ("maybe more than usual) bike runs great!

Welcome, or is it welcome back. I'll try not to confuse thing [as i do] before we even get started. You say head & tail came on, i guess you mean off the charger & possibly that's right for the tail light in the US but headlight is AC off the mag, regulated to a point by the battery & charging side which all sort of come from the same coil--or two lighting/charging coils in your case on the stator plate. If you meant they operated with motor running then ok i missunderstood. I'll leave it there as other with up to date knowledge on those will put ya right [put me right too lol].
Then i start again--bad habbit-- If you have a sealed headlight its best to add a single wire AC regulator as a bulb saver incase of a dry battery [acid old type] or blown fuse etc. Where i am in NZ the tail & headlight plus instrument illuminating bulbs all run off AC but i believe in the US the tail runs DC on certain models.

Long term memory recal--is there a gree/red spare there for a hgher rate of daytime charge. Or do you guys run with the lights on al the time. Also with the type of system it 'is', if you run higher wattage bulbs than factoryhe charge to the battery will be less. Conversly lower wattage give brighter lights [or too low & they'll blow out]. For-instance in a scoot with 25/25 say in an early V50 Yamy if we fitted 15/15 you'd blow it at revs.

Your right. My headlight runs off the lighting coil. If the bike is not running, it will not come on. The tail light will come on regardless as it runs off the battery. My biggest problem
is the battery doesn’t seem to be charging. Brake lights aren’t working now either. I’m going to have to start there. Maybe there is a short. 

Green/red is for night time. Light only comes on when I move the key to position 2. It’s still the original light bulbs on this bike and they all work.

When you're testing the resistance of the rectifier, red wire of the tester goes to the red wire of the rectifier, black to the white of the rectifier and I think that you should get about 5 ohms (but check this figure in the service manual). If you switch the wires around, then resistance should, I think, be zero. Full procedure is in the service manual.

When checking a rectifier you sometimes have a diode symbol on your multimeter  (    -I>I-       ) what it does is when you connect the two wires in one order it will show what's called the forward volt drop which is usually  close to  .5V  when you reverse the leads you'll get  no reading , this proves the diode is working allowing power to flow in one direction only to charge your battery

First making sure you have new battery now?

Second is sometimes added load/stress of bad battery damages rectifier.  Might be good to just replace it.

I got .5v and the little beep on my fluke. Nothing when reversed.

Ok, take what i suggest with a grain of salt as guessing a bit thinking back 50 years, [i'm 75] but ponder on the possibility. I get more radical down the end of the post lol.

Is Brake light bulb operating instead of the tail filament as has a high draw compared. Can you make green/red your daytime charge to boost the battery. On smaller bikes there was a spare wire or a double connector on one to swap a wires into the double for a daytime boost. Anyway yellow is highest output being for driving the lights so could take a branch direct off the yellow straight to the rectifier but battery might get too much of a hard time so then add a regulator & have plenty in reserve for the regulator to give as required. That's all assuming you're not using lights during the day as we don't have those rules but good for safety.