12V, 48 W,
P = V2/R
R = P/V2 = 48/(12)2 = 48/144 = 0.33 Ω.
So the normal maximum electric current is, 12/0.33 = 36 A
Now if it's connected across 230 V.
I = V/R = 230/0.33 = 690 A, which is a tremendous amount of current, and the car head lamp will blow up