To check the IAT sensor wiring: The red/yellow wire is the sensor's power, it will have 5V when the key is in the ON position. the other wire is the sensor's ground. check both wires with a voltmeter before you start changing stuff.

To fix the wiring: Run the red/yellow wire to pin C5 on you obd0 ecu, the other wire needs to be ran to pin c12. then recheck for voltage at the plug.

If the wiring is legit, and you still have no voltage at the IAT plug, then your ecu is messed up.

I about the shop but, If you can't fix simple problems like this... you get a from me.