For some reason the heaters on my Mjet have decided to only blow cold air now, It had been an intermittent problem where it had been blowing lukewarm air but now its just cold.

What could it be?

Coolant level is fine and no mayo under filler cap

radiator was replaced about a month ago after it split.

Remove passengers' side trim and touch heaters' hoses. If both are hot then you have a broken flap/actuator etc.

One pipe is hot and the other is stone cold?

When the blowers are on hot the window seems to steam up also?

You have a clogged heater core that it's leaking also.

Notice that it's leaking coolant! You have to replace it soon.