I would start by unloading the light circuit by removing it from the generator cutout or alternater connection if so equipped. It sounds like you have a short somewhere.... make sure your batter connections, battery hold downs, cables, starter connections... are all correct and nothing shorted to ground. I would guess you don't have a volt/ohm meter or are familiar with it's..... If you have a repro terminal box I would wiggle the studs and see if they are loose... or were warm from connection as the repros are a threaded screw from the back and come with a potential short issue new out of the box. To fix would require removal from the firewall, tighten the screw studs at the back ( terminal connections) and put some epoxy on the back side of the screwheads to fill the gap between there and firewall... this is insurance so it won't back out and cause a future short.
If there is not short with just the engine harness and instrument panel harness you have an issue in the light circuit harness/light switch.... my guess is it is elsewhere..
Of course ifyou have a volt/ohm meter you will find your short quickly.... Double check your instrument panel connections as well.
I had a house call I made yesterday to one of our local members and he said he replaced the ammeter but then it kept blowing the fuse afterwards. I pulled the instrument panel and he had removed the thin nuts on the repro ammeter and placed the electrical connections up against the case with insulators and nuts on top.. thus creating a dead short condition... the ammeter will never be the same!
Common sense... being alert as to your connections, soldering electrical, tinning wires, using heatshrink are all good electrical procedures to use.
Keep us posted on the root cause.
thx.
Larry Shepard