2003 GMC Sierra 4wd not working

Hi, I have a 2003 GMC Sierra 1500 that the 4wd doesn't work. Initially I replaced the 4wd switch panel because the lights weren't on at all. Once I replaced the panel the lights lit up but don't engage the 4wd, it just stays lit on the auto mode and when I press the 4wd buttons it flashes for a couple of seconds and then just goes to auto again.ive also replaced the 4wd actuator motor and still no luck.
Please advise