My '97 Subaru alarm started acting up a year or so ago. I first got under the hood and took the siren apart and taped off the two wires going to it. That helped the noise but like you, I still had flashing lights when the alarm sensor got tripped (and it would trip for no reason).
I finally crawled under the dash to the black box, which was the alarm module like Shifty mentioned. I was fortunate in that my factory alarm was dealer installed and the previous owner had kept the installation brochure from the dealer. So I was able to open up the black box and find some DIP switches and turning one of those off disabled the alarm. Fortunately my power doors and key fob still worked.
Maybe if you find the alarm module, there may be some instructions or something on a sticker inside the box.
My alarm box had two fuses btw.
Good luck - if I hadn't had the instructions, I probably would have had to go to the dealer.