aftermarket horns? interesting.. were those horns designed for the X5? this car has very sensitive electronics, with callibrated, balance signal and power lines - that means if the values of the replacement electrical component are not what the microcontroller is expecting, it will compensate by either increasing or reducing the power supply, but it can not overcompensate for grossly mismatched values.

Depending on the quality of the workmanship, the rain may have contributed to the issue - if the contacts were not properly insulated, or the system was not properly grounded.

There are multiple computers in this car, responsible for specific areas - the horns don't seem to be controlled by the computers though...

what I suspect, that
1. the battery may have been drained
2. depending where the work was performed, the steering wheel ring may have been damaged
3. as it was indicated before, ignition coil

if this is power related issue, then it could be the battery or the alternator... however, i would think, that the battery issues would be secondary to the horn installation - do you know where the replacement horns were introduced, and if there was any work done in the steering wheel vicinity?