If you are stuck at the reflective mirror section, try doing DNCLIP and goingthrough the hole at the firetruck looking area. I think i was stuck there too.

That's what I did, and from what I gathered, that's where I was supposed to go. I managed to complete the rest of the map with no problem...so...

I still want to know how I was 'meant' to do it though.

Sorry for the late reply...
If you look at the building which curves into the water on the farside the beach, there's some floating stuff and a downwards arrow, implying you can swim under water there. There's a locked door to a flooded room, and to open it you need to shoot a big switch in the room on the top floor from the jetty/pier.
Not the most straighforward thing, but I believe there's enough hints around, especially if you look at the viewscreens.

Later reply myself here, but I had the theory post-completion that the 'reflective glass' on the truck was meant to imply you shoot it with the shrinker, and then jump into the reflected projectile to walk right in.