i was also having same problem in my cuore. battery drain after night driving. one day i checked its wiring. it had wires fat like a finger going from battery and going to horn, headlights, radiator fan etc. coating was chipping off of wires at some places. tape used to connect was also going off. in short words, electricians had cut original wires and used heavy wires and relays to power fan(for working direct), headlights(relays and 90/100W H4 tubes), and horns. I myself took those heavy wires out and tried to use stock wiring with soldered joints. still i could not find shrink tube. also using stock 50/65 H4s and now my problem of headlights draining after using headlights is gone.
also when car is idling, check voltage at battery terminals. it should be around 14. then turn items on, one by one and keep checking voltage. if voltage drops below 12.5, you have a problem.