Where do you hook up a aftermarket oil pressure gauge on a 1989 4.3 s-10 Blazer?

The first is to remove the stock Oil sender unit for the warning light on your dash, most cars this sensor is not connected to your PCM and removing it will not affect anything.

You could also install a t-fitting that you can purchase from a local hardware store in order to hook up both factor and aftermarket senders.

Confirm this with your dealership or owner's manual before removing the sensor. If everything is ok then you can install the sender that came with the gauge.

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Hope This Helps.