Why that particular relay?
In all honesty, the only thing special about the defroster relay is the built-in fuse holder. If you are willing to use an in-line fuse holder, any standard 30A fog-lamp style relay should work the same if wired-up as follows:
-Most relays, horn ones being the exception, have either 4 or 5 terminals on them. Traditionally they are labeled 85, 86, 87 (one or two 87's), and one labeled either 15 or 30. The auto industry, or at least the European auto industry, standardized the circuit designaters for almost all cars in the 50's, so a 30 is almost always a battery (+), a 15 is a (+) when the ignition switch is on, a 50 is the (+) for the starter, and so forth regardless of whether you are talking about an Opel, Audi, Citroen, or whatever. Those designators stuck and are still being used today. This is all by way of explaining that you can buy a $4 relay and wire it in using the designators in the Opel wiring diagram and the markings on the side of the relay and it will work fine.
-Terminals 86 and 85 are for powering the coil in the relay. Direction of current doesn't matter between the two, so they are completely interchangeable electrically as long as between them is 12V when you want to energize the relay. Typically they are wired-up so the one is grounded near the relay and the other gets power from the ignition through a remote switch, but the Opel defroster is wired differently. In the Opel, the (+)12V comes from the output of the alternator via the same wire the idiot light on the dash uses.
(If the alternator is not working this wire has 0V, the idiot light will use it as a ground to light-up, and there will be no power to energize the defroster relay. When the alternator is working, there is 12V on this wire, supplying the power to energize the relay and keeping the idiot light turned off. This is how Opel kept you from draining your battery too quickly with the defroster on.)
The other lead on the relay will ground to the chassis through the switch on the dash, completing the coil circuit only when the switch is shut and the alternator is putting out power. The switch in the dash already has one side hooked-up to ground and the light on the dash should already be wired-up as well so if the wire above is connected to the empty terminal 86 on the back of you switch cluster you shouldn't have to do anything else for the control circuit.
-Terminal 30 (sometimes 15) is for power from the battery. The best connection is probably a big lug on the big post on the starter. This line is the one that will need a fuse installed. I recommend 16ga wire at a minimum for this and the wire to the defroster.
-Terminal 87 is the terminal that will now see 12V when the relay gets energized. Route a wire and hook this up to one side of the defroster grid. The other side of the grid hooks up to ground, typically a screw into the sheetmetal support around the window or below the rear shelf.
You should be able to see all the wiring in the wiring diagram you can download from OANA. Just add in the wires to replace the dashed lines on the diagram with wire and hook-up a cheap relay acording to it's own terminal labels and you should be set!