horn relay problem, 5 prong to 3 prong?

["Luke M. Kowalski" <interfaces@domain.elided>]

Argh....The horn is gone. I suspect it is the relay (I hear clicking
from it when I press the horn, and the fuses are OK). It went out when
I tried to make this woman in an Exploder realize that she was merging
into me on the San Mateo bridge.
This is in a 74 Spider. The horn relay is on the driver side of the
firewall, and it has 3 terminals. When I called my kind suppliers at
Alfa Parts in Berkeley (recommended, albeit the city council of theirs
bothers me) and ordered the relay, they said that Alfa now specifies a
Bosch replacement (the FAMM is NLA).
This Bosh relay has 5 terminals. I tired matching the little numbers on
the diagrams when hooking things up (36 to 36, 85 to 85), but no luck.

How do I figure out which of the 5 terminals used to be the 3 on the
old one? 
I have a wiring diagram for my car, and I am a member of AROC and ARA,
should any of these give me good karma.

thx in advance
luke, Belmont, CA

74 Spider (mute)
90 Porsche 911, 98 Audi A6 Q
Ferrari 250 GTE (after my wife finishes the MBA)
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