The GCR does not specify a location for the transponder, but here are some guidelines.
1. The location should be as unobstructed as possible (clear view of the ground).
2. If your transponder is hard wired, make sure you can see the pilot light to check to see that it's turned on.
3. If your trasponder is battery powered, be sure that it is in a place that is easy to get to, so that it is easy to install and remove for charging.
4. It's a good idea to locate it as far forward as possible, just in case you get into a situation like All Unser Jr and Scott Goodyear (just kidding about the Indy shot, but it was an unforgetable mess trying to sort out the margin of victory! It's the reason why SCCA uses transponders for timing only, not scoring!At least not scoring finishes!)
See you at the track, Jeff.