Mark,
Short on doing a thesis on this, you are correct. Wiring could be corroded and your aluminum car would be at risk. Monammonium phosphate and Sodium Bicarbonate have corrosive properites, but under the circumstance of a fire I'm persoanally not concerned at the time of corrosion. I think in an open engine compartment fire you will have a better chance in smothering the fire with an ABC type. Halon type you are exhausting oxygen, if you can't remove the oxygen, or heat you will still have fire.
You will also have all metal hardware for the attachment of this canister. I'm really not an expert on this but have been around enough aircraft and know that both are used.
Bill
P.S. By the way my name is spelled Bill Littleton
