[cardoc]

TSB 0905-02 addresses an issue with the alarm going off.

"If you have a 2005MY Legacy or Outback where the battery goes dead or the alarm system goes off for no apparent reason, check to see if the right (passenger) front door switch is making proper contact with the door. If not, the door may be slightly out of adjustment and need adjusting."

If the alarm is going off, that indicates the horn/siren is operating and doesn't need replacement. The alarm circuits need to be checked to find out what is causing the siren to sound. Start with the door.

Just in case you want to check what the dealer didn't or wouldn't, I have attached a wiring diagram for the security system.

I would also recommend that if the dealer service told you that the horn was the problem then I would find another shop for services. The horns either sound off or they don't. Replace them when they don't.

 

[cardoc]

If the alarm goes off for a long period, it drains the battery which lowers the available voltage to the horns, at which time you get the weak sounds.

Always start with the source and work your way out.

 

[cardoc]

The garage I bought the car from has agreed to pay for the full repair at a subaru authorised garage which is very good news. So it's booked into my local subaru garage to be done with replacement siren being fitted as they diagnosed. Then at least the diagnosis and repair are done by the same garage and I should not have to pay.

That's good that you get a cost free repair. But the horn doesn't create it's own power. There is most likely another issue.