First thing I'd do would be to see if any current is being from the battery with the car turned off, sitting dead. If it has an aftermarket radio, it will draw a small amount, >1/2 amp, to supply memory power.
If more than that, start pulling fuses and check as you go for current draw.
Another thing you might check are:
1. a ground wire from body to engine.
2. clean cable connection from eng to battery
3. clean buss bar at ext regulator at rad support
4. remove/rewire correctly wire harness to new shifter
5. Charging voltage at battery posts.
Gen light is supplied 12 v on one side from ign switch, the other side comes from the alt through the ext reg to supply 12v, thereby, if no fault w/alt, bulb doesn't glow. You don't nmention if you have ext reg or changed over to internal reg, but still, int reg alt has a terminal for gen light.
Others here probably can give more suggextions