I just tested this arrangement on the bench after I figured out the correct dropping resistor for the radio. The gas gauge draws around 200 Ma or .2 A and so a resistor of 30 to 35 ohms will do the trick very nicely. The rating should be greater than 1 watt, so a 2 watt one would do.
As far as your mentioning that something was attached to the ammeter, certain accessories were fed from the ammeter terminal using an inline fuse. No add-ons should ever be fed from there without a fuse.
