@Homebody: Have you ever used aspen damp? When I get dampness in my aspen colubrid enclosures (or they kick aspen into their moist hides), it molds quickly.
@sweetcastle: Simply letting the moisture evaporate for now, and not adding it when it is so damp next time you clean or add substrate, would help. Another thing to note is that using heat tape under damp substrate is going to evaporate all the water in the substrate that is over the heat tape, and do so pretty quickly. That can increase RH to undesirable levels, and also leads to crispy dry substrate in the warm area. Using a radiant heat panel instead of (or in addition to, in a taller enclosure) would avoid this to some extent; you may want to check them out. Reptile Basics RHPs are the easiest to get a hold of.
If a person actually wanted more ventilation, PVC is easily drilled/cut/routered to add vents. Keep in mind that this is permanent and you may regret it, and it may also lower the resale value of the enclosure. I use Animal Plastics PVC enclosures, and they ventilate pretty well -- but they have vents on the sides near the front, and the back near the left and right corners. Your particular enclosure may not have vents that are allowing any air to flow through, so that could be part of the issue.
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