Any general purpose rectifier should be fine (ex: 1N400x series). I used a 1N4005 equivalent part. It's rated at 1A (far more than the inductor kickback will produce) and 600V reverse voltage tolerance which means it should survive the nasty load load dumps and other transients that appear from time to time on the automotive alternator/battery supply rail.

I went ahead and moved the Fogs relay over since it's unlikely I'll need them anytime soon. For now, I just went ahead and installed the diode as pictured -- just without the solder (i.e. surface tension only). When I get some more time, I'll look for an easy and safe spot to install it. The wiring manual suggests there are a couple of junctions in that circuit. The A2 socket at the compressor clutch / sensor might be a viable option -- in this case you'd hook the diode anode to ground (UDPATE: In fact this is the proper solution) .

An obvious alternative would be to build a small harness including the diode to fit between the relay and the socket, using blade connectors.