When I start the car after it sets overnight everything seems normal. After shutting it off and starting it up a few times, it doesn't seem to want to turn over, or it clicks and turns over after a second or two. The voltage meter drops below the normal position after it warms up also. I can't run the car at night with the headlights or the heater because when I hit the brakes, the lights dim and the car almost stalls. I've cleaned the all the battery cable connections, bought a new battery, and had the alternator tested. I'm thinking it could be the starter or a bad ground. Thanks for your help.

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i would say a bad ground... check ur cables that go to the fuse block.

when starters start to go bad they constanly draw power from the car so it could very well be that or it could be a voltage regulator check and clean grounds one more time look for any shorts