That nipple is where the switching solinoids inside the truck (behind the glovebox) get their vacuum to open/close the doors. You should have a ported vacuum line going to that nipple. It really doesn't matter where you pull the ported vacuum from, but it was T'd off the cruise control hose from the factory.

Here's a pic for you:

[Linked Image]

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8