how can i fix my 4wheel drive for a 1999 chevy blazer?

how can i fix my four wheel drive? it had a light blinking when i tried to engage 4wheel drive but now i cant get the light on. i went to the parts store and tried a new switch with no response. i was told to check the transfer case or endoader(module) but i have no design or guied to work from. I need a schematic for the 4wheel drive.