I have a 2004 Ford Taurus. I have noticed that it has been making an odd smell. It smells a bit like a candle wick but strong enough to almost give you a headache. I have checked antifreeze and oil levels and keep them where they are supposed to be. The smell is only inside the car, I can’t smell anything outside. It seems to be doing more frequently as time goes by.
Does that mean that you have to keep adding antifreeze/coolant?
If that is the case, then it would appear that you are losing coolant, and that could be as a result of a leak from a hose or a loose hose clamp, or it could be the result of a leak in the heater core. I can tell you from personal experience that leaking heater cores are a known issue with Tauruses from that era, and I can also tell you from personal experience that breathing aerosolized coolant can result in some very serious respiratory issues.
IMHO, the first step would be to check for evidence of coolant leaks under the hood, and also to have a pressure test done on the cooling system. If it turns out that the heater core is leaking, I would suggest that you have it replaced a.s.a.p., in order to preserve your health and the health of your passengers. Just be prepared for a VERY large repair bill, as this is one of those repair situations where the part costs–maybe–$80, but the labor will run into many hundreds of bucks.
At this age another possibility is the valve cover gasket weeping a little oil that is getting on the exhaust. A little drop goes a long way in making a stink.
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Try removing blower motor resistor from the heater box.
https://www.rockauto.com/en/moreinfo.php?pk=1193999&cc=1425068&jsn=452
And inspect it for any debris.
Those springs you see are resistors.
Those resistors get pretty hot. And if there’s any debris touching them it can create a burning smell.
Tester
I would also suspect some overheating wires.
I friend of mine used to have a 2001 Explorer - not quote the Taurus, but also a Ford 
That thing was making that “candle smell” for some time and then resulted in smoke from under the dash.
We drove together on two cars to one destination and all the sudden he pulled on the shoulder and jumped out, so I stopped too.
It was a smoke from the steering column area.
We checked this and that, found quite nothing, other than wires smelled smoked, turned car on and it worked.
Trip was cancelled and in few days he became a happy owner of Toyota 4Runner he keeps for 10+ years already.
Makes me think maybe exhaust fumes are getting into the cabin
Might have bad exhaust manifold gaskets, cracked exhaust manifold, bad flange gaskets, etc., and it’s getting pulled into the car, when you have the ac on fresh air setting
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I have had that candle smell a few times over the years & it was always hot wire insulation, Plastic insullation smells like candles.when it gets hot
It does sort of have that burned rubber or plastic description. Good ideas above. Another one to consider is that the catalytic converter located under the car is getting hot enough to melt the carpet backing. Sometimes it isn’t the cat itself, but the cat heat shield will fall off when you hit a bump in the road and this happens.
I have replaced the blower fan resistor and the cabin filter. That didn’t help any, it still makes the smell. I guess the next thing I will try replacing the blower motor.
You could unplug the fan motor for a while and if the problem goes away, you have narrowed it down to the motor or its wiring.
While unplugged, if the smell persists, look and feel around for hot spots in the under-dash wiring.
I had this in a 1976 Honda Civic, before I knew much about cars, but it was easy to find the hot spot. A multi-wire connector to the left of the steering column was hot. Honda dealer identified the bad connection and ran that one wire through a new, separate connector.