when my cavalier gets warmed up..i step on my brake my oil light comes on,headlights go dim, and my heater slows down..

 

May be your alternator. Any other symptoms like interior lights going dim too?

 

I would agree that is likely a electrical issue, when you apply the brakes the tail lights are creating a current draw that is pulling down the system OR you have an electrical short in the fuse / relay box....
Might want to start with checking the battery cable connections and the connections at the alternator, check the ground cable for corrosion..

 

battery connection are all good..and so is the connections to the alternator

 

never noticed interior lights going dim