Well - quite strange. I never had this problem you're talking about. Someone should think a battery's ground should be the same thing.
But I've already seen some strange car's switching-logic. If you take for example the horn: On some cars both pins have +12V (measured to the car's ground and the car's battery ground) and if you use the horn then on of the +12V is taken to 0V so that the horn blows.
Strange things happen...

TeeMcBee
Got my Mk2! # 080000143