I have an '88 300SE with 250K+... still running great.

Today I noticed that while stopped in traffic my temp gauge kept climbing. As soon as traffic began to move, the temp went back to normal. I later pulled over and allowed the temp to climb while the engine idled with the AC on. I noted that the aux fan never came on. I later verified that the fan was OK by applying direct power.

My question is, could it be a temp sensor?? Do these fail??
Where is the temp sensor located for the aux fan??
Is there anything else that might the problem?
Maybe a resistor?
Any suggestions are greatly appreciated.