'Sfunny, but I was just talking to FarmerMatt about this very thing.
Apparently, the OBD-II control system is interested in the temperature of the battery, and considers this input important to control of the charging system. It will throw a code, and it will (I am told) also disable overdrive until it is corrected.
The battery temperature sensor is (obviously!) somewhere in contact with the battery - so start your search around the holddown arrangement and the battery tray. No, I don't know how much it coses, nor do I know if there is an aftermarket version available. I'm pre-OBD!
5-90