How do I open attic A/C unit to perform maintenance?
Do not just cut into the insulation board to inspect. At the most you might end up cutting some tape to free an access panel, but the main homeowner access is for replacement of the filter and that probably does not (and should not) have tape over it that has to be cut. 

My Carrier furnace and a/c evap coils are vertical so I am not familiar with a horizontal system. There will be a sheet metal access panel that slides up or to the side about 1/2 inch and then is released allowing it to be pulled out. Or there may be twist locks which free the panel. Do you know where the filter is? It could be that someone sealed the edges of the filter access panel with duct tape (or the metalized tape), but most likely the filter access panel will not be sealed with tape. Do you know how to change the filter on this unit?

I personally from your picture do not see mold. I do see rust on the bottom and this is somewhat concerning, but I don't see mold.

I am not familiar with this Honeywell switch on the top, but even in Houston (I live in Dallas) I don't think that one would need the blower fan to be on all the time. Google the override switch and see what you can find about the function.

Edit See http://yourhome.honeywell.com/en/products/ventilation/fresh-air-ventilation-system-y8150 It appears that this is for a fresh air addition to the conditioned air stream. I don't have such a system, but I have to wonder if this is will work in Houston because the humidity is so high there and the winter climate is so mild.