New starter / alternator, battery won't hold charge
1999 F150, 4.2L V6.
This past Tuesday, my starter gave out. My mechanic installed a new Remy starter and the wiring harness between the starter and the battery. The truck ran fine for one day. Then, while driving (fortunately) about a mile from my home, suddenly the radio and all of the instrument panel gauges shut down. The engine kept running and I limped home, expecting not to be able to restart the engine once I shut down. Unfortunately I was correct. However, after allowing the truck to rest idle for an hour or so I was able to restart it, but again everything shut down shortly thereafter and could not retart until after another "rest".
I noted that when I turned the headlights on and turned the crank, my headlights went out. I then suspected the alternator. I replaced the alternator yesterday with a new Remy 130 amp alternator. I was able to start the vehicle instantly, but now the battery light came on.
I checked the voltage on the battery with then engine off, and it was a little shy of 12V (around 11.50). When I turned the engine on, I noticed the voltage slowly dropping, which tells me that the alternator isn't charging. I boosted the battery overnight in order to get to 12 V, but noticed the same voltage drop this morning when I turned on the engine.
One thing I noticed is that the ground wire from the wiring harness coming from the alternator was not connected to anything at the time I lifted the hood after my gauges shut down (i.e. after new starter, before new alternator). When I installed the new alternator I made sure to connect this ground wire to a screw near the starter relay firewall.
Any help you can provide will be appreciated!