The one on the left is for out side the case to turn it on and off. The one on the right goes inside the case (it should already be there now). You would only buy that if the one inside the case is missing or broken.
Dave cant run the one on the left as my transmission does not have the plug for the switch so am going to buy the right. been talking to bowtie overdrive and they have told me what I need to do cheers.