hey guys, I wonder if you can help me out?

Returned to the car after a couple of weeks and it wouldn't start, enough power to open the doors and for the internal lights but not to start it.

Charged the battery overnight (until my charger said it was "full") but no change. Try to start the car and the battery light flashes on and off and the car makes a constant click.

Swapped the battery (which I know is fine) out of another vehicle (5.0L mustang) and this time no flashing battery on the dash but clicking again and the headlights coming on when I try to start the car - why would the headlights come on? Anyway, still no joy

Any suggestions?

Many thanks in advance