Coolant leaking vw Passat 1.6 1999 heat on with fan, window fogs.

Could be your thermostat... If defective, it will not allow the engine to heat up. Take the coolant cap off before you start the car when it is cooled off. start the engine and let it idle, look at the coolant. The coolant should not be flowing or moving right when you start the engine, if it is, the thermostat is bad. If you dont have enough coolant and you see it is low, this may also cause the heater to stop working. Someone may have by-passed your heater coil, check that. Sometimes when a heater coil leaks, people will by pass it so that they can continue to use the car or they cant afford to change it. It should be located on the passenger side under the dash or on the firewall inside the engine compartment on the passenger side. Another thing could be your climate control which is a fancy name for the heater controls on your dash, sometimes the switches become defective and will not operate the heater.

You have a heater core leak inside the dash behind the firewall.

My truck is in similar shape, you may have to use rad flush and make the coolant circulate better if it has rust blocking up the heater hose circuit. This is more common if the engine has been changed to one that sat for a while though. If your coolant looks clean check for vac hose couplers that may be off behind the intake manifold and inside the dash on the heater controls. Sometimes the multispeed fan switch melts inside and only works at 20% fan speed!, easy to fix with a good quality aftermarket control switch.

Start with engine cool, remove rad cap, start engine and turn heater on. Top up rad level and Let engine run until it get's up to normal operating temperature with cap off and add coolant as you see it go down. Watch in rad for coolant to start moving quicker, once gauge gets close to normal temperature, indicating that thermostat has opened. Wait until you see that 2 or 3 times, then top up and install rad cap. Top up coolant return bottle to proper level . You'll want to keep a close eye on coolant return bottle level for next little while. If coolant got that low in the first place, you have a leak somewhere that will have to be found.

test the heater core

If it is fogging your windows, you have a leaking heater core. Dash removal is required to fix it.

The only part that has coolant running through it under the dash, is the heater core, when it goes bad, you will smell coolant and the window will fog up, leaking heater core, replace it be for it start's to over heat your engine and cause more problem's in the long run.

maybe leaking heater core

the heater core could be leaking that explains the fog but i would have the water pump checked

the heater core behind the dash may be bad, it is the coolant tank for the heater system that is to say it is where the heat comes from. kinda pricey to have repaired at a dealership, if you can hear the fan turning but no heat, than it is a good possibility, or a vent hose has fallen off, get down and check under the dash