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I'd guess that the yellow wire might be for that purpose, so I assume you didn't get installation instructions with it. If there's a model number on it, it might be worth checking the Varad website and see if the instructions are available there.

If you have to do it with two leads though, it can be done. You could use a relay, and wire it such that the contacts would be held open while the ignition is switched on, closing when the ignition switches off, thereby completing the circuit automatically. You would just route either the 12V wire or the ground wire via the relay's normally-closed switched contacts. That's kind of a clumsy solution though.

If the unit is intended to be operational when the ignition is switched off then it presumably doesn't draw much current. If that's the case then there's a neat way to wire it, by connecting the 12V wire to a constant 12V feed, but then instead of connecting the ground wire to ground, connect it instead to the top end (the 12V end) of a relay coil, but it must be one which is fed with 12V any time the ignition is in Start or Run - you don't have to add that relay - I'm talking about one of the relays already fitted in the car. When the ignition is switched on, the 12V wire would then have 12V, and the ground wire would also be sitting at 12V (because it's at the top end of the relay coil) so there would be 0V potential difference between the two wires and thus the unit would be switched off. Then when you switch the ignition off, the top end of the relay coil would fall from 12V to 0V and would effectively become a ground point, and the unit would automatically switch on. It wouldn't be a true ground because you would have the relay's coil resistance in series, but for a low current device that isn't necessarily a problem; it depends on the unit's current consumption, and the resistance of the relay coil.

I'm on a new computer and I don't have all my programs installed yet so I can't see my wiring diagrams for your model, but when I get that sorted in the next day or two I'll take a look at them, and I'll sketch a couple of diagrams for those two methods, which will be easier to follow than the written explanation.