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Unless the charge controller does it (which it probably does), I'd stick to diodes. You only need one, and it's dead simple...as long as you get the polarity right. Otherwise, if it's dark out and the engine is off, it's going to discharge through it. Plus running a relay does take some power, if only a small amount, and if your solar panel isn't producing at that time, that's watts wasted.
Its not going to over-charge your battery if the solar panel is putting out power and the alternator is running. It will just lessen the load on the alternator. The regulators of both (assuming you use the charge controller) won't allow it.
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