One thing u can try is to get your front wheels (if it is front wheel drive) lifted on a jack or stand and then start it and drive it gradually to above 100 Km/h the speedometer will work as u r simulating a drive, but be very careful that ur car is on a firm stand with wheels lifted high enough from ground.
Then try switching on the light, deck, ac and then try honking and turn on indicators etc. If the problem persists, you should attach a battery load meter which will let u know that if there is something with the battery or generator. If you find everything OK and problem persists, then try to getting some real GURU for electronic system of cars.
You can also check the cooling system of ur car, bcuz sometimes on heating cars do things like that.