Information about CO leaks within the passenger compartment/cabin of the automobile

Open the hood, remove 2 plastic pins & cover just below the windshield on passenger side. There should be a preformed long rubber gasket that diverts any water away from the cabin filter (regularly replaced) found underneath in the compartment & sometimes the rubber becomes dislodged when changing the cabin filter.

You have a blown engine cylinder head gasket most likely.

If there is no visible leak outside in the engine compartment that might drip onto the exhaust mainfold to get drawn into the passenger cabin, the heater core is begining to leak inside in the heater plenum inside the passenger cabin..

The 2001 model Aztex has 2 cabin filters, they usually come 2 to a box when you buy them.
The charcoal carbon filters are the best.

Removal:

•Open the IP compartment door.
•Remove the rear inside IP compartment door.
•Open the HVAC module assembly access door
•Remove the RH passenger compartment air filter from the HVAC module.
•Slide the LH passenger compartment air filter to the HVAC module opening.
Remove the LH passenger compartment air filter from the HVAC module.

Installation Procedure:

•Install the LH passenger compartment air filter into the HVAC module opening.
Slide the LH passenger compartment air filter toward the left side of the HVAC module.
•Install the RH passenger compartment air filter into the HVAC module opening.
•Close the HVAC module assembly access door.
•Install the rear inside IP compartment door.
•Close the IP compartment door.

I Hope this information is helpful.

I am assuming that you mean that it is leaking within the passenger compartment. In this case, the most likely cause is a leak from the heater core. The only proper fix is to replace it.

The main thing you will need to do is to minimise the amount of 'excessive' engine fumes being generated in your engine compartment which in turn will 'leak' into your car compartment: 1) Degrease your engine and check for leaking oil gaskets especially around the oil refill area: oil leaks at the back of the engine falling on your hot exhaust pipes and on your engine block will generate lots and lots of 'nasty' smelling fumes! 2) Check that your exhaust pipes joints are well sealed especially the one contained within the engine compartment: taking great precautions, check for the obvious leaks from smoke emitted but you could use soapy water to assist. 3) Check that the lining in your engine compartment are in good condition taking particular attention to the 'Firewall' ones (e.g. the one between the engine compartment and the passenger side compartment). 4) Check for broken heating/cooling air hoses leading into the passenger compartment.
Hope it helps!

there is a black tube in the pass side engine compartment behind the acumulator drier
(silver can) needs to be cleaned out

The fuel lines are steel except for the plastic ends where they need to be flexible for the push-on type connectors. If I am right, those plastic ends pass just under your cabin air intake. Just below that, is a common bend where the steel lines usually rot. If you have high miles they may be baked, or if you had recent engine work they may have been flexed to one side hard enough to split one the three. If one is leaking it is best to replace them all. Don't take chances with gas fumes or leaks. Do it right away!

This sure sounds like the AC condensate drain is plugged, if it is allot of condensation water builds up in the heater/AC case and leaks in on the passenger side, the drain is located on the pass side under the hood down low on the firewall, it is most cases it is a rubber nipple about 1/2 inch in diameter sticking out of the firewall with a little flapper valve in it, stick your finger in it or a stick and see if water drains out.

Many auto cooling/heating systems use a "blend door" which mixes both hot and cold air to provide air at the temperature selected with the temperature control. When the temp control is in the full cold position, the blend door moves to a position which allows only cold air to pass into the cabin/passenger compartment.
That blend door is generally operated with engine primary [unmodified intake manifold pressure] vacuum. IF there is a leak in the primary vacuum line system, then at low engine RPM the leak may bleed off the vacuum necessary to hold the door in the cooling position. Then when driving, the vacuum will be much higher, "over riding" the leak, and allow the vacuum motor, which operates the blend door, to hold the door in the proper position and provide only cold air to the cabin/passenger compartment.