Mold/ Mildew smell from heater vents

every time that i turn the heat on (espically the front drfroster) i get hit in the face with a wave of moldy, musty, mildewy smell. its almost nausiating (i hope that inhaling that is not bad or harmful by the way...is it?). So what should i do to get rid of this smell.
Would I have to pull the dash apart to clean out the tubes and vents or is there an easier way.

I also have that leak on the passenger side. which is a hole in the firewall i guess i read. so where exactly is that and what should i do to fix that

I have had mold smell from the AC in other vehicles that I was able to cure with an excessive dose of Lysol sprayed into the intake of the heat/AC system, with the fan on full blast to circulate, then on low to get it to build up in the system. I'm talking 2 cans of the stuff, and the vehicle was NOT driveable the rest of the day, you would choke on the fumes in the passenger compartment. After that, the mold smell was gone.

There is a seal between the firewall and the AC evaporator/heater core that is common to leak, and let water in on the XJ, unfortunately the fix is to pull the dash and heat/AC core to replace the seal.

Another possible cause is just a clogged drain for the AC evaporator. Water builds up in the box from condensation on the AC (remember the AC is on during defrost to remove moisture from the air, so this can happen in winter as well) and it will spill out during driving when the water sloshes above the non-sealed gaps in the heater/AC box. This promotes growth of fungus/mold as well.