Mercedes GLS Bumper Antenna Problem

That's a strange set of symptoms. I had a similar problem with my old BMW, though it was the car antenna on the roof, not in the bumper. The car radio reception was terrible, and the keyless entry was acting up. Check all the connections you can find related to the car antenna. It could be signal interference from something else too. When you mention the engine won't start without the key, that makes me really think it's an antenna issue. Is the car radio also acting up?

The car radio appears to be functioning normally. However, where exactly should I be looking for these connections?

Okay, if the car radio is fine, that points more directly to the keyless entry/start system's automotive antenna. With my BMW, the car antenna connector had corroded where it connected to the receiver. I don't know the specifics of your GLS, but I'd suggest taking it to a workshop. They should be able to diagnose it properly. It could be antenna replacement or just a bad connection, but messing with those systems can be tricky.

Thank you for the insight. The issue was indeed a faulty connection at the bumper antenna, as suspected. The repair cost 95€. Your experience was helpful in guiding the diagnosis.