No, next to the top on the right side is the AM/FM antenna, the one in the middle of the trunk lid is the XM antenna. The XM tuner is in the trunk too.
They probably did a coolant change on it. If it wasn't done correctly, you will have to "burp" your car. Search the forum, there's thousands of posts on it. Also ddmworks.com will have information on how to burp the car. After that's done, see how it does. I would recommend doing this at least twice if it still continues. After that it's time to look at more expensive repairs.
Did you do the WP? Is it burning coolant (usually will get white smoke out of exhaust and it should smell sweet)? My other question, you said you're calling for heat when it heats up that much and you aren't getting any heat....right? No oil in the coolant or vice versa...right? Are you tuned? If so, they might have your timing advanced too much causing the car to heat up....just a thought. The other issue you could have if you're tuned is that it doesn't go into "fuel shutoff mode" during coasting down. Hope I've given you somewhere to start.... Not sure if you've used the coolant bleed technique or not, but thought I'd suggest it.
A blocked heater core will not cause the engine to overheat. It just means that you will never get warm air for yourself.