I am not sure about this stray current malarky.
Currents tend not to be stray, they flow from one source to another, taking the easiest route.
You cannot measure the current in the coolant by placing a probe in the water. This will only give you the potential difference (voltage) with relation to earth. If you put 2 probes in the water, a set distance apart, measured the potential difference and knew the resistivity of the coolant you could calculate the current.
I think what you are worrying about is galvanic corrosion
. This is where you have 2 disimilar metals, add an electrolyte (in this case coolant) and you have yourself a nice little electric circuit. Electrons will flow from one metal to another and erode the softer metal. This can be speeded up by the presence of an external electric source.
Really, the only way to stop this is to design it out of the system by insulation, or use of a sacrificial anode
to protect the components. As Pete says, the OAT coolant contains corrosion inhibitants to slow the process down anyway, I say slow it down as you will never stop it.
You say you have had one radiator corrode? How old was it?
Aluminium cannot rust as rust is a byproduct of ferous metals.
What is it with all this addition of negative cables and stray currents in coolant? You aren't doing a course in theoretical electrics are you?
Cars have been functioning perfectly well for many years with stray currents, no negative cable and galvanic action. Yes you will get corrosion, but I think you will get an ulcer worrying about these minor problems before your car dissolves.