I tried with a slim jim but so far no luck.  IWhat's the trick?  It's a 78 coupe.  Both doors are locked all windows are up and latched.

If you can't get it with a slim jim, there's a kit with a hand-pump air bag, a wedge, a long rod with a hook, and some other accessories you can use. This is the kit I have and it's worked on everything I have tried it on. 

 

Use the wedge to pry the top of the window with care, then install/inflate the airbag to make enough room to manuever the hook to grip and pull the lock tab by the interior door handle, then remove the rod, airbag, and wedge, and open the door like normal.

-Bill