I assume that the light popped on automatically at 90K miles or some higher
multiple of 90K.
I believe that the reset mechanism on the '89 will be the same as my '91 pickup.
You must remove the instrument cluster and transfer a small screw on the back of
the panel from one ground connection to another.
My truck has 215k on it now and I've had to do this twice - once at 90K, again
You can find the details in a good repair manual or at alldata.com.