I think that if you remove the headlight assembly you will see where the threaded legs of the mount are located and with a 10 mm socket you can remove the nuts and pull the unit out through the top....
remove mirrors (pull rubber to show the bolts). remove windshield garnish (flat screwdriver between it and the headlamp)- remove headlamp lower trim (two philips screws)- you will now see 4 bolts holding the headlamp.
You'll need to keep you're radio bracket in there as it supports the guage cluster. Alternative is to buythe bracketfrom an Interstate model. It gives you more room.
If you haven't already purchased aradio or if you find, which I'm sure you will, that a CD system just doesn't fit...Most GL12 owners have gone to the SD card-reading radios. The radios also usually have USB and i-pod inputs. Pyle makes really short ones. You'll find that very important, but look for any unit under 5" long.
I installed a Blaupunkt (6") and it was a real challenge, even on the Interstate. I had to cut away most of the bracket.