I may be out of line chiming in with such a respected, knowledgeable group here, and I'm sure if I get shot down, I deserve it. It seems to me, though, in my meager understanding of electrolysis, that adding an anode is merely fixing a symptom rather than fixing the problem. If you have more than 75mV across the coolant to a ground, you will have corrosion problems if there are aluminum parts in that coolant. It's because the coolant has become, or started out as acidic. It usually comes from exhaust gasses getting past the head gasket (CO2, NOx or SO2) and turning the water acidic. It can also be dissolved salts in the water, which is why even using distilled water can fail...the salts are added to the water as they leach from the engine and contaminate the distilled water.
The way these anodes work is they provide a very reactive surface, more reactive than aluminum, and therefore they will corrode first, allegedly saving your aluminum. However, I have to point out that modern coolants have an anti-corrosion package included with the other chemicals in the coolant. If the anode is used, the anti-corrosion chemicals will do their best to protect that anode, but will quickly be depleted and now you have nothing left to protect your heads or other aluminum. Additionally, the very small contaminants from the corroded zinc or magnesium anode will now be circulating through your coolant, and will most likely adhere to the hottest surface they encounter, the small passages in your aluminum heads. That rod doesn't just disappear. It goes somewhere, there's no black hole in your cooling system. What goes in will be there, maybe in a different form, but it's all there. Many of today's modern engines have aluminum heads, for example the Corvette and Camaro. No anodes there. I may be wrong, but I don't think any OEM in the US uses an anode.
Don't get me wrong, if you have water conducting electricity in your cooling system, it's a bad thing that you need to fix right now. But adding an anode is like putting a band aid on a bullet hole. Fix the problem that's causing the electrolysis.
Anodes are for boats, where you can't control the conductivity of the water coming in contact with the metals. You should be able to balance your PH and get the relatively small amount of water in your cooling system under control.
At least that's my understanding of why you don't want to run an anode in an automotive application.