Thank's folks - I'll try both the passenger armrest and the center console (although I have a set of cupholders in the console).
I tried the dash compartment - but the unit didn't beep - which means the array doesn't see it. FWIW, FasTrac's computer system is archaic - they've sent me several "notices of violation" when the system couldn't read the transponder. They can't seem to match up the plate with the ones they have on file - although other times they read the plate off the camera and put that transit in the "read from the plate" column on the bill.