You could attach an anode to the frame of the vehicle and that would provide you with some corrosion resistance. However, you would have to attach the anode in several locations around the frame to provide any real protection. Also you would have to "break the seal" of the corrosion protection that is already applied to your frame. I don't know if it would work for body panels because of surface issues. Most automotive paint finishes are good enough as is. You might be able to attach anodes in older cars where rust is beginning to show, but again you would have to put the anodes in several different locations.
Years ago I read something about an active electrical system which was supposed to completely prevent corrosion at the molecular level. Haven't heard anything in at least a decade. Either quack science or suppressed technology.
The following is a quote from Corrosion Doctors regarding those systems: "Countries like Canada and the U.S.A. have actually got court orders to stop these products from being sold - simply because they don't work."
Sacrificial anodes are all over your car already. Galvaneel (like NickE mentioned) is a zinc layer that is on any good car body (I won't mention the names of the companies that don't use it!)
For the sacrficial anode to work, you need ELECTRICAL connection between your anode and the part you're protecting. (Don't mount it to underbody PVC!)
Also, be mindful of whatever you use to strap it on. If it's metal, it will also corrode! Some plastics don't hold up well to temperature changes.
The most common sacrificial anode used in the automotive industry is zinc (hot dipped, zinc plating, galvaneel, etc.
Be careful not to create any crevice corrosion cells.
In the end, I think you might have a fun project, but I think you'll be disappointed if you do a Cost/Benefit analysis.
It doesn't work because an anode must have an electrolytic solution of some kind to work, in other words water. This provides a path from the anode to the metal being protected (the cathode). It's a simple DC circuit.
