[Merc6]

Welcome to the forums, They didn't test the battery cable before replacing the radio? That usually is the issue when someone posts this here.

 

[Merc6]

Re seating fuses as in "remove and put back in the same spot you took it out of" will not void the warranty. Just leave the car off and doors closed(keys on your person) for this process unless you are a LTZ.

 

[Merc6]

LTZ = push button start. The car would sense you are near and activate a few items that you would be pulling the fuses for if you had the key on your person.

I'm still leaning towards the battery cable if they didn't check the battery for a bad cell.