If you have an aftermarket head unit, you simply run the spaker outputs from it to your subs, unless you already have them running to your stock speakers. If you don't have an aftermarket head unit, the only thing you could do was take the wires running to a set of stock speakers, and run them to the subs instead. Of course, the stock speakers won't be used then, and the subs probably won't sound that great.
No other way than to get an amp. If you took them from a home, then they are more than likely 8 ohms, and headunits are designed to run a 4 ohm load, meaning you're only getting 1/2 of the power that a head unit can put out, which is VERY weak.