I have a weird issue that started with a battery change oddly enough. Immediatly after changing the battery, I lost all instrument panel lights. The wife left lights on one day and the battery died again, A jumpstart/recharge later the lights worked again and now the horn doesn't work. The only common denominator I've found between these two items is the BCM. Is there a way to verify this?

 

Those things may not be related. Check the horn circuit for problems before assuming it's the most expensive item.