heater doesn't work during winter time but now it start work at +12 temparature.

Most likely you have a partially plugged heater core. There are various things that can cause your heat to feel less hot but the most common cause is a flow restriction. Your best option is to try and run a cleaner through the cooling system and back flush the heater core until the fluid coming out is clear and clean. If you do not know how to flush a heater core look on YouTube for videos from creators such as ChrisFix. There you will find very informative videos on how to do it yourself.