you can always remove the radio, look behind it and track the wire that plugs into the Sirius port. The wire is fairly short and will be connected to a silver and black box about 3 inches square. This is your tuner and the ESN will be on it.
Did it on my '97 262 with no problem. Just make sure you get the correct diameter gages and that you keep track of all the wires. I did one gauge at a time. There are some good deal on Ebay for Monterey gauges under "Monterey Boats".