no bass with hatch open

I got my receiver, amp and subs installed and went to tune/set the gain today (see my previous post). When I turned my system on with the hatch open (it's in a 1985 Camaro), I barely had any bass. I kept adjusting the gain, but I had to turn it all the way up to get anything. I noticed that the subs weren't even moving that much. I shut the hatch and sat inside the car, I could see the subs moving much more and could hear a lot more bass. Probably still not as much as two 12" subs should produce, but more. Not sure if this is a common issue or not. I guess my questions are (1) how do I go about setting the gain properly if the subs are so quiet I can't hear when they start clipping and (2) is it common to have that little bass with the hatch closed versus open? Thanks.

Yes, they fire up into the glass.  That's good to know.

As for the second part of my question then, I can't get to the amp with the hatch closed so is there another way to properly adjust it?  With the hatch up, the speakers don't move much or put out much sound so I'm not sure how to tell if they are clipping.