Car will not start dead battery

In last two months four times I've had a dead battery. So I put on charge but for a reason the trailer light is flashing and wont go off, but when I turn ignition try see if battery charge then trailer light stops flashing but only thing that comes on the dash is sbs but still car wouldn't start but clicking noise. So turn ignition off and trailer light back on flashing on it own and can't switch it off. Appreciate it so much as I have no idea and don't want buy a new alternator or battery without idea if it something else. Thanks