98 Tacoma 4wd will not engage

My 4wd indicator light will not come on and front diff will not engage. I have read about the vacuum lines to the actuator issues, but the only line to my front diff is the vent tube. (dont think mine is an ADD model? ) How does this diff in my 1998 engage ? I have manual hubs, manual tansmission.
thanks in advance

 

You are correct you don't have ADD.

 

How does the diff engage? What can I do to diagnose and fix problem?

 

What engages a non ADD front diff. I have a manual transmission and manual hubs. The indicator light does not come on and the 4wd does not engage. Is this non ADD (1998 tacoma) system electrical ? were is the switch, fuse or other device to engage diff?