Thanks, I'll definitely keep the grounds in mind. Is there a way to check with a multimeter fairly easily instead of following wires in the engine compartment or under the car somewhere?
My buddy is coming over this evening - we'll try the battery from his car in the Taurus just to see what happens. At least this would be a no-cost way of proving that the battery is or isn't the culprit. The garage told me this morning that it's normal to see a bit of acid on top, I am not convinced.