On my car the transponder is mounted at the rear of the frame. I have seen at least one other picture there. Is that required? That is the only reason I would think someone would locate it there, but I do not see it written.

That is a great question. The original timing results of the Sebring race showed Brian winning but that was because he had a forward mounted transponder. Soon enough that was corrected by visual at T&S.

The location is NOT specified by SCCA.

We put ours on the inside of the right rear chassis post that the toe link is attached to because back in the day that is where it had to be for the Pro series. It is hidden from most impacts and is close to the master switch. The winner of the race will always be called visually but that is not to say that anyone would pay attention to finishes behind that. In a points race that may mean the difference between 3rd or 4th. Have you ever had to chase that one point at the end of the year to qualify for the runoffs?

Of all the things that should be spec in a spec series, shouldn't the transponder location be spec? (Mine is also on the very rear frame. I always thought that was an odd place for it, but assumed everyone else had it there too.)

As Steve mentioned, the pro series had a required location (RR frame member) but club has never done so.