I'm having issues with my 2005 F150. The dome and bed light come on by themselves draining my battery. Took it to the dealer, they stated my alternator was bad. Replaced the alternator and starter. Checked vehicle grounds and they all seemed fine. Problem continued. I'm hitting a wall and dont want a new alternator, starter to get ruined and maybe my battery. Any help would be greatly appreciated.

 

Sounds more like a door switch. Are they same lights that come on when a door is open?

Hard to believe somebody diagnosed that problem as alternator related. They must have focused on the dying battery, and ignored the lights, in order to sell you a new alternator. Not sure I'd go back to that place.

 

I think the headlight switch can be set to not turn on interior lights when the door is opened, try that if the problem goes away then the system thinks a door has been opened, bad switch or wire, the
mechanic you took it to is a moron.

 

First, I do not know the answer but I spent some time reasearching the associated circuitry. There are two fuses involved with the following discussion 15A F24 and 15A F6 and the power from these fuses eventually pass through the Instrument Cluster. F6 is the power to the headlight switch (then to the Instrument Cluster) and the Vehicle Security Module (VSM). F24 feeds power the Battery Saver Relay (it needs to be energized) and then into the Instrument cluster.

Power to the Dome light and Rear Cargo Bay lights as well as the interior lighting is supplied the by Battery Saver Relay circuitry. The Battery Saver Relay is controled by either the instrument cluster or the VSM and my searches never did find which one actually was the controller. The contacts located inside the Battery Saver Relay have to be closed (relay needs to be energized to close the contacts) for the Power to the Dome light and Rear Cargo Bay lights as well as the interior lighting to illuminate. The Battery Saver Relay is designed to remove power to the supplied loads if the doors are left open for an extended period of time (best I could determine this was about 10 minutes although I also saw references that said it could be up to an hour) to prevent the battery from going dead.

Assuming you have ruled out the dome light switch and door switches then MY GUESS is that you have an Instrument Cluster or VSM problem that is causing the Battery Saver Relay to energize randomly. It does not seem like the Instrument Cluster would have timing functions built into it but the VSM apparently does. Again, I could find very little info on the VSM but it does tell the Alternator what voltage to be provide and I suspect it also controls the Battery Saver Relay.
CAUTION Do Not remove the VSM without first saving the settings into your diagnostics since they need to be reprogarmed back into the VSM or it will loose it settings.
The fact that the dealer changed the alternator makes me think the VSM could be the issue, I hope not.
If it were me I would take it back to the dealer (or find a very very experienced shop) and have them read the DTCs associated with the VSM (they generally start with B.....) and go from there.
I suspect the reason that I found so little detail in regards to the VSM is that modern diagnostic equipment electronically spits out the information without anyone having look any further. I also suspect that the diagnostics (in this case) pointed to an Alternator over voltage and the dealer went no further with his diagnostics.
Please provide feedback when the solution to your issue is found. My head hurts!