Hey Domisvo, there are plenty of threads about that in this forum, this one for example. Instead of fragmenting them by asking ten-fold answered questions again without the well-known basic knowledge as basis for asking questions, I highly recommend using the search function and reading the threads first!
If you would use the search function, you could see this problem is very well researched and there are numerous proven solutions.
There are Nissens and Thermotec heater core products with connections to the pipes inside the car rather than all in one piece. That saves us the enormous effort of taking the entire dashboard and heater unit out of the car just to replace the core. Do read this and the other Threads for more info! Replace the coolant with G30 (easily available in Greece) and don't use the old OEM stuff! The OEM coolant was the underlying cause of this problem.
Possible core replacements amongst others are:
Thermotec D65003TT
Nissens 707183