Before spending $200 for a tech, swap the relay for free. The relay box is above the fuse box under the dash and it's wide open. Just pull out relay #10 (the horn relay) and set it aside. Next, pull out relay #12. That one is for the fog lights and it's the same relay as the horn. Plug the relay you pull out of the #12 slot into the horn relay slot #10. Next plug in a new fuse and see if the horn works.
If it does, take the relay #10 you pulled out and set aside to the dealer and get a new one (or two or three, it's a very common relay in the Boxster). Either swap the test relay you used from the fog lights back or just put the new one in.
If the horn doesn't work properly after swapping the relays, put it back like it was originally and get a tech to look at it.
I hope it's just the relay.
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