Hello,
MB stopped coating the evaporators in the a/c box with anti-fungal coating
in 2000. However, there are a few things that can cause your issue as well.

- Blower fan speed set to min or low. This causes moisture to never be dried
out of the heater box. Try turning your fan to max for a few minutes with the
a/c turned off. This should help dry things out.

- Version coding in EIS control unit wrong. Have your dealer check the coding
in the electronic ignition switch for climate control settings. The RE-HEAT mode should be set for "Anti-icing Protection".

- Heater box not draining evap water completely. I have had to replace a few heater units (complete) for being cast funny and not draining well.
Also, the drain tubes should be checked for being plugged.

- A/C air filter plugged. causing weak air flow. Should be replaced at every B service.

The dealer should invesigate these items and proceed accordingly.
Good luck,
Dr.D