Well, the horn is actually a "weapon" that has no ammo. So I would think:
vehicle fire "weaponname"
would work. The actual names of the horns, taken from the bis's commented game config, are:
CarHorn
TruckHorn
SportCarHorn
There may be more horns in custom addons, or in o.pbo (the stuff added by OFP:R). However, this might work in the general case:
vehicle fire primaryweapon vehicle
Or perhaps:
vehicle fire (weapons vehicle select 0)
Of course, those likely won't work if the vehicle has real weapons too...