For several months my charging gauge read at or near the red-line. Over the past week it is almost pegged-out if no lights or heat is on (when lights and heat is on it drops to 16-17, but still in the red). I replaced the battery and altenator but the problem still persists. All other guages appear to be OK. Where else should I look?

Check to see if you have an in-line regulater. If not you might consider installing one.

You may want to check the wires at the back of the alt. make sure the regulater wires are not grounding out . If they ground out that will send the alt. to full