Hello. I'm installing an Intellitronics GPS unit to drive the speedometer in my 88. According to the wiring diagrams I've found, I believe I should clip the brown wire going to C1 in the cluster and attach the output wire from the GPS unit to it. I see there are VSS high and low inputs as well. Any help is appreciated.

Welcome to the forum!

Why do you want to drive the speedo with the GPS? What is the issue you are trying to resolve?

The brown wire from the speedo cluster is the VSS output to the ECM.
If the purpose is to drive the speedo with the VSS signal, then you likely want to connect it to the VSS HI (Yellow) wire at the 203 connector in the console area by the ecm. If a Fiero VSS sensor (or other GM VSS Sensor from the same era) is not supplying a signal, you almost always need to install a speedo circuit to change the signal format to one the FIero Speedo can interpret.

You probably need to make this circuit and use the GPS output signal as the ECM output signal in the picture.

GPS to drive any speedo will cause problems and maybe legal problems when car is sold etc.
Consumer GPS speed and distances often isn't reliable, either short term, long term or both, no matter what hype is pushed to sell them.
Dash Speedo and it's Oddo can be wrong and car sold like that but must be reported as Unknown Miles.
That kills value to sell a car. If isn't reported then can be sued by buyer and/or state can come after you.

If not careful, GPS will kill the car battery when car is off.

VSS drives the Speedo that drives the ECM. ECM can/will trip DTC 24 not seeing speed "data" but may not set the code and still cause problems.
ECM Can Not be driven by VSS w/ above method. ECM uses 2000 Pulse/Mile but VSS output is 4000 P/M.

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