First....get yourself a good copy of the 400F's electrical diagram.
Just like you, i was an electricalnoob, so this may not be the most direct troubleshooting of an electrical sytem, but it got me to the point where i knew the layout of my system, and, i knew that all my connections were good and solid. as you go along, you'll find areas that need attention. dont skip a thing.
Take out your battery and put it on a float charger.
I started within my headlamp.
each electrical connection i took apart and made sure was cleaned of grime,gently scuffed for good connection, dielectric greased, and insulated well when put back together. resolder anything that was soldered, but looks weak or corroded.
Keyswitch, take it out, open it up and clean out that black goo. regrease and reassemble. check wires for corrosion, melted sleeves, and shorts. properly replace any that need it.
do the L&R handlebar switches.
check all grounds at battery, frame, and marker lights. clean all block connector pins as best you can.
voltage regulator
out that melted fuse holder take and splice in a new 15A blade type.
do a search function here for charging system troubleshooting, and walk thru the steps to see if your v-reg is charging properly. your v-reg can be gently tuned. should also be in your shop manual.
probably enough to get you started...
