I put new intake manifold gaskets on my 89 IROC. While re-assembling the upper intake, I notice I lost a wire. This wire plugs into the bottom of the plenum (towards the back) and runs back to the firewall. Once it hits the firewall, it plugs into the wiring harness. In total, the wire is probably 6 inches long. It also seems that its some type of sensor. What does this wire do? Where can i get another?

 

could be some sort of intake manifold air temperature
or something to measure the air temperature.

 

It sounds like that's the exact wire you're describing. If so, it's for the IAT sensor, or intake air temp. You could probably buy a new pigtail one to splice in from most parts stores, or there's always the junkyards. TBI cars are all over in most yards, and they use the same wire and plug for the coolant temp sensor on the intake manifold. In fact, the last time I bought a coolant temp sensor, it came with a new pigtail and plug.