Assuming the temp gauge is correct, and the engine is indeed warming up (the gauges in later cars, 240 and 740, don't read the actual temperature, they just go to the middle and stay there unless the car overheats) the heater valve is the next thing to check. The hose I was talking about is a small vacuum hose, about 1/4" in diameter. It should be connected to an even smaller metal tube (1/8", maybe) on the side of a round capsule about 1 1/2" in diameter that is part of the heater valve. The heater valve also has two large water hoses (1" outside diameter) connected to it. If the small hose is connected properly, there are two other likely possibilities: No vacuum (usually a problem with the heater controls in the dash) or the heater valve itself has gone bad (which it can do without causing a coolant leak--it just doesn't open any more) You will need a helper for part of this next test. Find the heater valve. Check the vacuum hose. If it's OK, start the engine and have your helper watch the heater valve while you move the heat control knob back and forth. You should be able to see movement of a small lever arm on the side of the vacuum capsule. If no movement, pull the small hose off and see if there is vacuum, which should vary as the knob is moved. If there is vacuum, try to move the arm on the valve by hand. It may feel springy, but it should move somewhat. If it is completely jammed, you need a new heater valve.(a further check of the heater valve is to run the engine for a while, with the heater on full, then feel the two large water hoses that go to the valve. They should both be hot. If they are both cold, or one is somewhat warm, the heater valve is not opening. If both are slightly warm, but not hot, it is time to take a closer look at the thermostat.) If the lever arm moves, you may be able to temporarily tie or wedge it in the open position, so that you can have heat while you troubleshoot the vacuum problem. Be careful not to damage anything if you try this. You don't really want to leave it like this, as the heat will be on all the time which makes the car slow to warm up. I have found on mine (with everything working properly) that it warms up A LOT quicker if I leave the heater off until the gauge gets at least half way up to the normal position before I turn on the heat, which is usually only a couple miles. I can then turn on the heat and warmth actually comes out the vents. If I start the car with the heater on, it takes at least twice as long to feel anything, because the engine has to heat both the water in the engine and in the heater at once. With the heater off, only the water in the engine has to be heated, which happens quickly, then you turn the heater on, and hot water goes to the heater, and the cold water that was in it goes to the warm engine and is rapidly warmed. All of this takes place before the whole system is up to operating temperature, at which point the thermostat should open.

Did the no-heat situation happen suddenly, or was it a gradual thing?