Well, first off, I will say, I'm just a ham not an ee, so take this for what it is. Think of it this way. It is called a ground plane. Like a stick poking up out of a pond you get a reflection that is the opposing image of the stick. The lower the frequency the bigger the plane needs to be to give you a full image of the other side. For 2m You only need 10 or 15 inches but for 28mHz you need a bigger pond, if you will, to reflect it. It's not about electrical conductivity like in a DC circuit. It's about making it look like a plane from the feed point. So if the roof rack has a high impedance connection to the roof, the roof does not act as part of the plane. That's ok for 2m cause the rack is big enough to reflect the image. If you tie one corner of the rack to the roof you would get some reflection from that direction but most of the image is still only reflected by the rack in the other directions. It will work, but it will be compromised. Add another connection and it will give you a more complete reflected image. Two more after that and it would start to look like a full reflection. So think of it not as an electrical connection but as a reflector of you antennas radiated signal. Ideally you want it in all directions. Four is just my suggestion for a minimal approximation for infinity. It's not a magic number. Two would be kind of a compromise. Now that said, all this is just theory. Try it. Two may work just fine for what you need. As I said very few CB are set up to work well but they are still useful for vehicle to vehicle communication within a trail group. I have my HF antenna mounted to the front of the hood on my jeep. I have no doubt it would work better were it mounted in the center of my roof. Even with copper tape running from the feed point out under the hood and tying into the body it is a compromise. In the 7mHz range it gets even worse as even if you do get the entire vehicle acting as part of your ground plane, it still is not big enough to reflect the entire image and you begin to require a capacitive connection to the ground under the vehicle to get the rest of the signal. At that point the conductivity of the soil becomes an issue. I'm kind of rambling now but I hope that helps a bit.
