Heator blowing cold air at highway speeds

[Elyaouti800]

1984 Mercedes 300td

This has started happening about 2 months ago never really paid attention to it until I was on the highway. The car would be 80 degrees Celsius blowing hot air on the road until I would merge in a highway going 60 miles an hour the heat pump blow warm air. I also noticed a puddle of coolant on the floor the next day coming from the passenger side. Can someone give me some insight what could be the issue. I checked the coolant resivor and it was low so I topped it off with more