2002 Hyundai Elantra Problems

Hi, I recently bought a 2002 Hyundai Elantra. I knew the possible problems with the used car when I bought it, but some of the things that worked fine during the test drive seem to have gone awry the very next day.

The radio on the car did not work right from the start. There was a lot of static and crackling when I test drove it. When we tried to switch it on after the sale, there was a crackle and it died. Few minutes later, the lights on the dashboard flickered and came back on. We noticed the interior light did not work now. The keyless entry feature also seem to be affected although it worked fine earlier. The moonroof feature worked fine. The engine died the next day at a stoplight. I had to restart it to get it going. I am not sure if any electrical malfunctioning was triggered when the radio died or if its just separate fixes and would like to be advised on what needs to be checked in order to fix the problems.