Our 27 year old apartment complex has fiberglass shower surrounds that become moldy at the seams. I have been advised that the seams are caulked in the back during installation so it is unnecessary to caulk the seams that show an obvious crack vertically at
The corners or horizontally between the top of the tub and shower surround. This doesn't sound right to me because construction caulking after all these years could be faulty and water is going to cause dampness in these crevices where mold will grow. Please advise.

Caulked from behind during installation, not likely. You want your caulk to be on the surface to prevent infiltration. Clean the cracks with a mildewcide and recaulk with a good bathroom caulk. DAP sells one with a mildew preventer built into it.

I have a fiberglass bathtub/surround and joints have been caulked. We can not control the mildew joints even with constant bleaching, and both tub & surround are very dull from constant scrubblng
Should we tear out and replace tub and surround?
Can we reglaze?
If moisture is behind the caulk (where tub and surround join) how do we get it out if we want to reglaze?