Targa uses four analog frequencies in the VHF-hi band. Which channel is in use in which location changes each year and could even change on the day of the race, if conditions require it.
Don't look for a Targa license in the Spectrum Direct/TAFL files - the radios are rented from a St John's communications company each year.
I can't give you the exact frequencies as I have been involved with running Targa before and will be in the future, just not this year (because of other commitments) and handing out the frequencies would be frowned upon. But if you search and/or use close call, you will surely find them.
As far as I know none of the competitors' cars use any radio communications. The only other thing you might find in use is FRS for some marshals to talk to one another.
One other thing that I've seen done in the past is the frequencies assigned to towns for their own use (fire department, public works, etc) are used when the town is preparing to close the roads and such. For example in Clarenville, the fire channel was pretty talkative before and after the stage.
As a rally operations person, I only ask that if you do figure out the stage ops frequencies, don't blare them at top volume all across the land, and don't repeat or act on what you hear on there. It's important to the safe and smooth operation of the rally that comms are not leaked out, or situations are dealt with by somebody with a scanner. Case in point - several years ago a car crashed on a stage, as happens once in a while, and someone who heard about this (I don't know if they had a scanner or not, could have just been a resident nearby) took it upon themselves to call for an ambulance. Targa already has an ambulance on standby that they dispatch through their own command structure. So in the middle of running the rally, this extra ambulance, followed by rescue and RCMP, roars up looking for a big disastrous crash. This caused complete confusion and concern until it was figured out that the very minor incident, which Targa's ambulance and emergency response crews had already dealt with, was the only incident on the course.
That having been said, do enjoy the week, and I wish I was there with you, especially with that 60s/70s GMC pickup running (the first pickup truck in Targa)!