MG Car Repair: Horn and head lights, fuse holders, lanexa virginia

Question
Have a 71 mgb. Just recently the horn will only work if the headlight switch is on. I cleaned the contacts on the steering column. When I ground the pencil with a wire the horn blows as long as the light switch is on. Won't blow if the ignition key is on. All fuses are ok but have not cleaned the fuse holders. Also the map light will only come on when the head light switch is on as well and not when car is running or ign switch is on. I have the schematics in color and can see no correlation to the headlight  and horn other than they share a fuse. In reading a lot of your Q&A's I know I will need to start at one end of the car and check everything but was hoping you could offer an opinion to what might be going on?

thanks in advance,

John
Lanexa, Virginia  

Answer
Hi John,
You don't have to do a lot of testing for the horn circuit because it is a "Stand Alone" circuit and the horns are powered by a purple wire from the fuse box and has nothing to do with the head light circuit. The horn push switch is a ground for the horns (purple with a black tracer wire)
A simple test with a test light on the purple wire will confirm power and a simple ground wire to the purple w/black tracer wire will confirm the grounding circuit.
You should use a wiring diagram to see what the headlights do to make the horns operate and the use of a 12v test light. If you don't have a wiring diagram for your car let me know and I will put one up on my web site for you to copy.
Howard