I get lightheaded from the smell/fumes put out by this heater.

The heater will burn the fumes off , how long it will take I don't know.,can you put it out side and let it run till the smell goes, never spray any thing on the heater elements you could have a fire.

the heater draws air from outside the engine compartment, usually from the grille between the hood and the windshield. check to see if the carpet is wet on the passenger side floor, under the glovebox. I suspect you have a leak in the heater core, and you are getting coolant fumes rather than exhaust fumes. Other signs of a heater core leak would be oily film on the inside of the windshield near the defroster vents, and low coolant level or overheating. If you do actually have a leak in the exhaust manifold, it could be cracked, or leaking from joints in the pipe or where the manifold bolts to the head. If the leak is at the head, you will hear a sharp hiss every time that cylinder fires. You might also be able to feel the hot gas escaping, if you (obviously use extreme caution to not get a severe burn) put your hand near the manifold. If exhaust fumes are getting inside the car, you should also look for unplugged openings in the firewall. The firewall should be as airtight as possible so you should not smell any fumes even if there is a leak.

This is a somewhat common problem on Nissan products.
The V6 engine has several small rubber hoses connecting the fuel lines, and when it is cold weather the hoses leak fuel.
The heater - defroster picks up the fumes from the outside vent under the windshield. When you are moving the fumes flow under the van.
If you can find the leak sometimes you can just tighten up a clamp or two.

You would have to contact the makers of the heater to give you a clear answer on that , most heaters smell when first put on from new and you would have to burn this off first ,

Fumes are bad, stop using it until a technician has checked it out. Get a CO detector!

Yeah, and it probably smells like evaporating anti-freeze. Turn off the heater to avoid the smell. Good Luck.

Have it checked properly - sometimes the reg won't open fully, but the heater will still run, but with improper combustion resulting in a funny smell.

You might have a slight odor of gas when the burner first ignites. Some gas does escape before it completely ignites. It shouldn't smell after 2 years of use. If you can see the pilot flame burning from the top of the heater, the heater pilot assembly is pushed over too far. The pilot fumes could be escaping in the room and cause a smell.

Generally a kerosene heater will put out a strong smell for a few minutes after it is first lit, and for a few minutes after you put it out. Generally, it will be much stronger after it's extinguished. I usually extinguish mine and immediately set it out on the concrete porch to keep the smell out of the house.

Opening a window for a few minutes right after lighting the heater will help with the fumes at startup.

If your heater is constantly putting off fumes, even after burning for a little while, then it may be time to burn the wick dry. This should be done every few months anyway as general maintenance.

First, siphon all the fuel from the tank. You will need to disassemble the heater and gain access to the top of the wick. Raise the wick to it's highest point and use your fingers or a flat nosed pair of pliers to break up the hard black tar coating on the top of the wick. The smaller you can break the coating, the faster it will burn off. Once thats done, reassemble your heater, raise the wick to it's highest point and light it. It will probably burn for an hour or two because it will first burn all of the fuel soaked up in your wick, but then it will burn off the tar. Finally, it will go out.

After this, you should be ok, to refill your heater and relight as normal.

If you find when you disassemble it that the wick has a hard tar ring down inside, you may have a hard time breaking that up and cleaning it. It is possible... But I would probably just replace the wick.

If it has not been run for several months it could be dust burning off.