Here is a 1980 Honda Express NC50 Wiring Diagram (pg. 29-10).
Here is the rest of the 1977 - 1981 Honda Express Shop Manual.
If the brake light blinks a little bit with the signals, that's normal (my 1981 NC50 does the same), because it's a 6V system, and it doesn't have that much power. Urban Expresses and Express SRs had 12V systems later on in 1981-83.
You might want to get some halogen turn signals (very bright), a 12V car taillight (works perfectly and easy to find, and I think the one I have is a 1157 taillight), and a removable headlight bulb assembly (not sealed beam) if you don't have one already. No special bulbs are needed for the signal and high beam indicators, and the speedo, just the generic ones (look for the number on the bulb).
Also, if you don't have a new battery, get one. If you don't have a good battery, you can blow the headlight and the other lights won't work right. If the fuse is damaged, got a new one. The battery is the regulator, so if it is weak or dead, there is nothing to regulate the amount of power going to the lights, which means they get surges of power way greater than 6V and blow out. And lastly, make sure all the wire connectors are in securely. My turn signals and ignition switch wires came out at one time, causing them not to work.
I hope this helps quite a bit!