Working on an 02 Pontiac Grand Am with 3.4l V6. When I check the battery voltage at the battery while the car is running all I get is battery voltage. I replaced the alternator thinking it was the culprit, and made sure the battery was fully charged. I had the old alternator tested twice, and both times it tests good. Is this a common problem, or do I need to do a bunch of wire chasing to find the root of the problem. I'm not much good with wiring, but with good directions I can figure it out. Thanks.

That has a poorly designed wiring system. Check for recalls on Ignition switch routing.
Otherwise, check both ends of both cable to be certain they are clean and tight. Voltage drop test the alternator and don't forget to look at scan data. You may want to know what the computer is seeing.