How about using RJ45 connectors? My Guino is on a breadboard now. To connect to the car, I put an RJ45 jack on the breadboard and found an RJ45 terminator at Lowes for the other end. I made the cable by crimping RJ45 plugs on each end of 8 conductor telephone wire. So I've got 8 wires to work with and a simple way to connect to them elsewhere in the car.
Here's the terminator:
(UPDATE: These connectors are called RJ45 Keystone Jacks. Good price here
http://www.monoprice.com/products/su...05&cp_id=10513)
The connector comes with a little plastic punch-down tool. I used it to push 3 solid copper wires into place. In this case, I connect to the wires with alligator clips. It makes a very neat installation.
There are RJ45 jack templates in the Eagle libraries. Jacks can be had for $0.85 at AllElectronics.com.