How do you wire car so that low beam stays on when high beam is engaged There are 3 wires A B and ground. A is low beam and it cuts out when B which is high beam engages.?

You should consider this carefully because you will be drawing twice the current from the headlight circuit.

If you decide that you do need to make this change, verify that the feed wiring and fuse/relay will handle the additional current.

You can then either install a diode that allows power to pass from B to A, but not back or wire a relay in with the coil energized by B that provides power to the A lead.