Model A Ford back firing and jerking after lights turned on

[Rata Road]

A friend recently finished a restoration on a 30 Coupe and we went for its first decent test run yesterday.
Was buzzing along great about 45 mph when we turned the headlights on the engine started misfiring really bad with massive back firing etc. We managed to just limp home, jerk jerk backfire, dogs barking and people coming out of their houses....funny.
On inspection the aftermarket Fuse fitted above the starter is blown hence lights, brake lights etc not working.
It has a new wiring receiver thingie at the bottom of the steering column which has some issues as the horn can go all by itself and since the light switch comes down the same system perhaps the horn and this problem are related.
I can understand the fuse effecting the lights etc but what is the link to this and the running of the engine? The problem occurred exactly when the lights were turned on and stayed.
Any suggestions before we investigate would be appreciated.
Thanks in advance