[MetroMPG]

The idea (electrically driven rear axles in FWD car) comes up a lot, but I've yet to see anyone implement it, DIY.

I did speak to a guy who did it for fun with an old Tercel 4wd wagon, but it was basically a science project in a junkyard car; I don't think he ever drove it on the road.

JG's right: you'd need much more than a relay. Don't forget batteries too! Either heavy and bulky (lead acid) or super expensive (Lithium).