Start with checking your speedometer cable at the transmission, it's going to be located on the driver's side in the trail-shaft housing. If that's good, then it's going to be a good chance that the speedometer may have gone bad. There are two places to check, the speedometer cable it self and the speedometer. Check to make sure the cable is not damaged or disconnected at ether ends, if that checks out good then it's going to be a bad speedometer it self and will need to be replaced. Good luck josh, i am not sure but check also to see if it's a electrical VVS sending unit. If there is no speedometer cable then your going to have an electronically controlled speedometer and look for loose or damaged wire harness, but being a 1989 it's going to be mechanical. 1990 was when they started doing the change over to the electrically controlled speedometers in the GM line of cars and trucks. Good luck and keep us all posted.
