Actually you can remove the harness from the cluster! If you look at that pink handle you will see that it is being held in place by a little tab, Just press down on the tab and slide the handle as far as it will go and it will pull right out! Nissan also uses that type of connector!
How did you wire it for your alternator to work? Mine is a 87 GT and the instructions say to wire in a bulb with a resistor to pin 14 light green with red trace and my wires in pin 14 are blue with a red stripe…
You should really make sure that the GPS antenna is not blocked by anything, For optimal performance it really needs to be mounted without any obstructions! It will function properly if you were to mount it on the dash!
Why waste the money on the GPS kit, You can access your vehicles VSS wire directly behind the kick panel on the driver side, You do not have to get under your car and access the transmission! All the dash needs is a signal from the VSS sensor, Which you can access directly behind the drivers kick panel! I’m going to go out on a limb and say that you are not a mechanic and most likely learned from watching videos on RU-vid!