When i put on my headlamp the horn will blow and the light goes off in my 1999 leganza what could be the cause

Something that fuse powers is shorted to ground . Need to look at wiring diagrams , service repair manual , power distribution diagrams , disconnect everything it powers - one circuit at a time till fuse doesn't blow any more . Then find the short , videos on youtube showing how . EGN A ???? IGN A fuse . ENG 1 or 2
Underhood Fuse-Relay Center Fuse/Circuit Breaker
Rating
Description
ABS Fuse M4-N4
60A
Electronic Brake Control Module
A/C Fuse K11-L12
10A
A/C Compressor Clutch Relay
AUX A Fuse P7-R7
30A
Upfitter Provisions
AUX B Fuse P8-R8
30A
Upfitter Provisions
BATT Fuse M7-N7
50A
Instrument Panel Fuse Block
BLOWER Fuse M3-N3
60A
Blower Motor, IP Fuse Block
ECM-B Fuse F11-G12
20A
Fuel Pump Relay, VCM, PCM, Fuel Pump and Engine Oil Pressure Gauge Sensor
ECM-1 Fuse H7-J8
20A
Ignition Control Module, Ignition Coil, Camshaft Position Sensor, VCM, Fuel Injectors, Crankshaft Position Sensor
ENG-I Fuse L8-K7
20A
Heated Oxygen Sensors, Mass Air Flow Sensor, Exhaust Gas Recirculation Valve Solenoid, Evaporative Emission Cannister Purge Valve, VCM, Secondary Air Injection Relay, WATER IN FUEL Indicator Sensor, Fuel Heater, Glowplug Relay, Wastegate Solenoid, Crankshaft Position Sensor
HORN Fuse H11-J12
20A
Horn Relay, Underhood Lamp(s)
IGN Fuse F5-G6 (Diesel Only)
20A
PCM, Fuel Solenoid Driver, Engine Shutoff Solenoid
IGN-A Fuse M6-N6
40A
Starter Relay, Ignition Switch
IGN-B Fuse M5-N5
50A
Ignition Switch
IGN-E Fuse F7-G8
10A
A/C Compressor Clutch Relay
LIGHTING Fuse M8-N8
40A
Instrument Panel Fuse Block, Headlamp Switch
LH-HDLP Fuse F1-G2 (Export Only)
10A
Left Hand Low Beam Headlamp
LH-HIBM Fuse F3-G4 (Export Only)
10A
Left Hand High Beam Headlamp
RH-HDLP Fuse H1-J2 (Export Only)
10A
Right Hand Low Beam Headlamp
RH-HIBM Fuse H3-J4
10A
Right Hand High Beam Headlamp
RR BLOWER Fuse K5-L6
30A
Auxiliary Blower Motor Relays
ABS Relay (Expot Only)
--
ABS Warning Indicator Lamp
A/C Relay
--
A/C Compressor Clutch Coil
A. I. R. Relay
--
Air Ijection Reaction Pump Clutch Coil
Fuel Pump Relay
--
Fuel Pump
Horn Relay
--
Horns
Starter Relay
--
Starter Solenoid

Of couse there is a fuse , plus a relay ,wiring , switch in the steering wheel , horn slip ring etc... Testing at the relay could be done if you knew how. . If the fuse was blown it would be because of a electrical short . Fuse don't blow for no reason .
VEHICLE RELAYS Operation Diagnosis

horn relay located on the right side kick panel,you will see a row of relays,if horn relay is okay, more likely your horn control pad has a short inside causing horn button contacts to stick causing horn to keep blowing.

I have a 1999 corvette and the horn will not blow when pushing on the horn pad. It beeps when locking the car and security system.how do I locate the problem. Thanks Bob

could be you might check it out. many items take the same relay so you might try exchanging it with another one with the same numbers. If that isn't it you may have a short in the horn itself.

Well, it could be a fuse. Check the associated fuse, if it is ok then proceed to checking for power at the horn (it should have 12v). If the horn has power then replace the horn. If it does not then seek proffesional certified help due to the fact that the problem is going to be in the steering column or horn pad, and that will require dealing with the airbag system (which is NOT recommended) unless qualified.

Maybe horn became unplugged. Put 12 v power to horn make sure horn works. Put test light on terminal on horn have someone press the horn button, if light comes on and horn does not blow you may need new horn. if light does not come on you may have bad contact at horn button, blown fuse, broken wire, bad horn relay or possibly just a bad ground.

I've had this exact problem before when being jumped off. The fob DOES NOT make it stop. We removed the fuse that goes to the horn. As soon as your battery gets recharged from the alternator, the fob will work again and the alarm will quit sounding. The only pain about removing the fuse is that the cruise control will not work until you put it back.

Check in the Fuse Relay Box under the Hood for Relays that Correspond with the Problems and Replace them, A Bad relay could cause the Blowing Fuses Problem also.

you may need to put a relay in it as the current from the horn may be back feeding up the immobiliser horn circuit and causing a short. Sounds like the immobiiliser has its own source of power seperate from the factory circuit When you press the horn the power goes to the horn and up the immobiliser horn circuit and it blows the fuse.