Won't start one day and fires up first crank next day

...................And while we are "on" all this, it is VERY common for voltage drop through the ignition switch circuit. What this does, other than robbing voltage when the charging output is low, is to actually cause OVER voltage output from the alternator, because the VR "thinks" the battery is low. It is a wise idea to check this, and if it exists, fix it or workaround.

One way is to install a relay. Trigger the relay from the "run" wire, break off th eloads, and power the loads from the relay contacts

TO CHECK

Turn key to "run" engine stopped. Measure between the ballast "run" terminal and battery PLUS post. You are hoping for a very low voltage, lower the better, Anything over about .3V (3/10 of one volt) you should go shopping for the cause.

Same on VR ground. Check it with engine running simulated "low to medium cruise" RPM. Stab one probe of your meter into the top of the battery NEG post, and the other into the mounting flange of the VR. Lower the better, zero is perfect VR MUST be grounded to battery NEG