Terrestrial repeaters are usually physically located at the highest point in a market. I know for a fact (I know the person who is contracted out to do the maintenance on them) here in Buffalo area, the repeaters are located at the top of our tallest skyscraper downtown, One HSBC Tower.
I was assuming this, but do you have the navigation center? If so, press the 3 and 6 buttons down simultaneously, you'll get a hidden diagnostic screen, there is an option called 'SDARS Signal'. Unfortunately it won't tell you if your receiving the signal from the satellites or the repeaters, but it will give your current signal strength. 0 = None, 1 = Poor, 2 = Moderate, 3 = Perfect. It would be interesting to see the value when you experience dropouts at a red light. I wish the diagnostics were more in-depth. On the old old old XM Delphi receivers you could press a numerical sequence and it would give you the real time BER% (bit error rate) for the two satellites and the repeaters and other geeky stats. My diagnostics on my Sony Home Theater Receiver with Sirius built in even has two signal meters, one for Sat and one for Terr.
Newer Chrysler products are the only vehicles I can think of of the top of my head that have SiriusXM units. There may be more, I'm not sure. All these years later after the merger, auto manufactures are still for the most part using the system they launched with. GM, Toyota, Nissan, Honda and Hyundai are XM. Ford, Daimler, BMW, VW with Sirius. All of the imports that Ford used to have partnership with (Mazda, Jag, et al) are still Sirius, as far as I know.