1990 Chevrolet Camaro RS overheating

I have a 1990 Camaro RS V8 305 and it overheats at idle. It drives fine and stays cool on the freeway but waiting in traffic it starts to overheat, I replaced the thermostat but the problem persists. I noticed that the rad fan doesn't switch on, I'm not sure what temperature it's supposed to switch on at but it wasn't switching on, does anyone have any idea what it could be?