Service Vehicle Soon light?

Hi everyone, I just purchased a 2001 Impala LS, with the 3.8 I had heard they were great engines and a reliable car. Im hoping they were right. I am however having a issue with the Service Vehicle Soon light. The only time it comes on is when I turn on the headlights. So I figured it was a bulb, I replaced all the light bulbs from headlights to the small 194 side marker lights and it still comes on. Last night I put the vehicle in reverse and the wife noticed the reverse lights were not on. I then went to Oriellys and bought 2 new bulbs, replaced them and it still isnt working, next I replaced the fuse and relay in the fuse box on the passenger side. It still comes on and the reverse lights dont work. Anyone give me a slight hint here. Thank you

Welcome to AF, get the service vehicle soon stored code, you need to get the car scanned for this code.