Easiest way to check the wiring is to warm up the car first so it's got some heat that would normally show up on the guage then just plug in your spare ECU, switch the ignition on, if the temperature guage works and the HRW works then it's the ECU if it still doesn't it's the wiring
If it is the ECU then I wouldn't attempt to swap the NEC chip you might just end up with two ECU'S that don't work, I'd do as Brian suggested
Russ