I believe the horn lives behind the n/s headlamp assembly and you are right it is a separate unit (VSS). For the time being the alarm is behaving itself and it's too friggin cold to start investigating at present. As for auto electricians they believe Grand Voyagers are the Devil's work and would prefer to stick their hands in pre heated frying pan and I can understand why.
(Page 1257 of the RG-Body.pdf manual shows the location of the alarm horn)