97 Geo Prizm

I have a 97 Geo Prizim which 2 weeks ago my starter shorted out and caught my wire harness on fire, my mechanic cut out the burnt wires and reconnected most of them with connectors then taped and put cuduit back arounf them and my car was running great for about 45 min. I came back home shut the car off for about 15 min then as I was driving about 3 miles down the road my speedometer started not registering and then my car lost power and I pulled off the road. Then when I went to restart it my battery was dead. I then recharged my battery went and had my alternator checked and my battery both were showing a good charge. My mechanic thinks what happend was I left my radio on and drained my battery. I am scared that my car will lose power again as I am driving it