i was wanting to know if i need to run a fan on the motor with my gtb and ss13.5 brushless motor to keep it from thermal shutdown in my RDX touring car???

no no no no not enough draw from motor to make it heat up

You probably won't need one. I ran one last weekend in a TC3 and the hottest it got was around 125. The weather was cool (maybe 60 degrees), so I tried gearing up a tooth or 2 every round, and eventually got to a point where it apparently reached the limit of available torque and went slower. It still didn't get very hot. The final gear ratio that seemed to work the best was 5.6 to 1.

I would and do run a fan. Now that it is getting cooler it might not be needed. Just what gear ratio are you running on RDX and the size of the track?