I'm not sure if the sedan has a 'diversity unit' like the wagon but if so track it down and check the connections. In the wagon it's located in the left side C-pillar. Pull each connector, spray with contact cleaner and reinsert and repeat. Another simple antenna test is to hookup a test antenna to the back of the radio. This is something the radio shop should have done before selling you a new radio. Be sure the test antenna is grounded. If you decide to buy a cheap antenna for the test you'll also need a DIN to RCA adapter assuming you are buying antenna for US radios.