Aww you got the super rare 89-91 tach, you need to go back and grap the rest of the wiring . the 92-93 trucks use a pre-wired system because they have a crank sensor, the 89-91 trucks that didn't have the crank sensor the tach was a whole kit that came with wiring and crank sensor and all that.

It's the same tach as the 92-93 trucks but the plug is just different

 

Dang I paid 120 for mine

 

i'll have to go grab the wiring harness the next time i go down if its not crushed already....wish i would of grabed it now...