If it smells sweet, and rather sickening. You may have a leak in your heater core. (In the winter when it is cold, your windows will get all fogged up inside)
probably the catalytic converter- or you child is smoking in the car
Defective blower motor?
I would suspect you have a fuel leak at the fuel tank. Can be a fuel line leaking or the fuel pump gasket may be defective.
If the fuel injectors are dry then the 1998 Honda Civic may have a gas leak. The car should be inspected, even a pin hole leak can lead to a fatal accident.
Defrost uses the a/c, if it doesn't smell the same with a/c only on. The smell is coming from the blower motor. Bmw's for example have a problem where the blower blade contacts the plastic housing and causes it to rub/melt.
The Charcoal Canister which is located on the engine fire wall just under the control box and fuel filter needs to be replaced as it can no longer store after burn fuel.That is why the smell is more intense after driving and worst if the blower fan is running.
Malfunctioning emissions system. Get this repaired. Engine probably running rich from a leaking fuel pressure regulator. If there is fuel in the vacuum line the regulator is bad.
My 98 Civic LX radiator last around 119,500 miles. When it leaks, it will smell like burn rubber and battery acid through the air vent. It showed no sign of any problem before that and can be driven for about 70 miles at freeway speed with no problem, though not recommended.
The clogged cat and the foul smell have the same cause. This cause is probably a misfire but could be caused by burning fluids that are not a suitable fuel. It is perfectly normal (on any car Honda or otherwise) for an odor to occur when you continue to drive it when you know it is not running normally.
The catalytic converted has melted, buy a new one.
you have a bare wire touching on your blower motor
Bad catalytic convertor. Egg smell is a dead giveaway.