I just rewired my ammeter on my 82, I don't think they changed it much so should be the same.
The shunt is located out in the wiring harness by the voltage regulator. The 2 taps are located in 2 different splices in that same general area. One on the main lead coming off the alternator and the other in a splice in the main lead going from the voltage regulator to the power distribution system. They are splices 201 and 202 on the 82 but that is probably not the same on an 89.
As long as your charging system is working, I would definately leave that for last. If your wiring behind the instrument cluster is hacked up then digging around for the correct splices and the shunt won't do any good.

 

Could well be, I spend most of my time in 80-86 and am going by memory on the 87+ models.

I think it was in 1987 where they also reversed the resistance/gauge reading, IOW low resistance = low reading on the gauge until such point where it went high resistance = low gauge reading.

Look up the senders on an auto parts web site, some of them tell you the resistance ranges.

 

For the gauges, yeah, you're probably right on the shunt's location, I think I was remembering the clusters that only had the red light and not a gauge....

 

Awesome info guys thankyou very much. I'll get some pics up of this endeavor ASAP.