A/C Troubleshoot Assist

So my 2002 Civic's A/C was intermittent for the past few years, either working great or not at all.  This summer it's been in the "not at all" category.  I popped the hood and turned the A/C on and the condenser fan doesn't spin.  Pulled out the relay and jumpered it with a paperclip and the fan runs like a champ.  But the relay's not bad; I swapped it with the horn relay, which I know works.  So I checked with the relay out and there's no voltage difference between the two control pins regardless of the state of the A/C pushbutton, so whatever's supposed to be pulling the relay in isn't doing it.  The light for the A/C being on is illuminated, meaning the car does think that it's turning the thing on.  Any advice what to check next?