There must be a line in the /etc/inittab which starts the graphical login manager.
In my system the said line is
Here in my system the file 'prefdm' is a shell script which starts the prefered display manager.
U may check that script (if urs is also similar). U will surely find something useful there.
I also feel, i could start the display manager without using that script. If i change that line like
the following it should work also; but not sure
[ i think my gdm path is that]