i have a 0.7V source that is coming out of another circuit (car controller) that needs to be sinked. If wire is simply connected to ground the doors will lock. If the wire is connected to a 470 Ohm resistor the doors will unlock.
For the sake of argument, we'll assume that what you're doing is OK.
What's the other end of the resistor going to? If it's Ground then leave that connected, and then...
Get a 5V "reed relay", one with a coil resistance >=
170Ω (Radio Shack has a good one, but get a diode for that, too), and connect one of its contacts to the 470Ω junction and the other to Ground.
[The relay switch will short out the 470Ω when the coil is energised.]