Not always true. I had a bad starter on my 91 F-250 351W and it would drain the battery as soon as I shut it off. I put a new battery in it thinking it would fix the problem. It didn't do the job. As soon as I put in a starter, it never did it again. That was five years ago and still starting awsome!

 

Personally I would look for a hood light, alternator, starter solenoid or a starter itself. These will cuase an overnight drain for sure.

 

thanks for the replies guyz.
after i posted we had two 6 inch shows, no heated garage and temp didnt' get above freezing for about 2 weeks.
It's finally warmed back up. Had to deal with locked up water pump on another vehicle while it was so dang cold.

Anyway,
Underhood and glove box light do go out. I can actually see them out when almost closed.
fuel pump: i can hear it run when the key comes on but not when i reconnect the neg cable(with key off).
Voltage regulator: manual says it part of alternator
Alternator: disconnecting three prong and one prong plugs won't dim test light
cigarette lighter: wasnt' even hooked up
headlight switch: when disconnected wouldn't dim test light
radio: vehicle has no radio. Checked those wires for bare ends, found none.
starter relay/solenoid: unbolted it from ground/fender to see if it was bleeding thru. don't think so. (not very conclusive, i know)

decided to go back to power distribution box under hood.
[pulling the headlight fuse dims test light.]
Wiring diagrams in manual weren't very helpfull. Decided to turn power distribution box over and remove bottom cover and see what were the wire colors away from the headlight fuse.
the thick brown wire opposite the power side of headlight fuse is joined to a small brown wire leading to a 15A fuse on the fender side of the PD box
Here it gets interesting. this time i discovered that if i pull that fender side 15A fuse, test light dims but it also seems that power is bleeding back thru the light green and yellow wire which the book says goes to DRL (Daytime Running Lights module)

So maybe i'm getting close. Now if i can figure out if vehicle actually has one and where the heck it is.
Component locator doesn't help. Anyone know where it's at?
Would this DLR be the same device that turns off headlights automatically after key is off? Would it be the same as a headlight Sentry/Sentenial? need help with the name, not sure what it's called

 

ok, followed the LG/Y wire to a plug that was plugged. No DRL.
Also found out the only reason the testlight was glowing bright thru the headlight fuse and the other 15A fuse was because of the underhood light. If i pull the underhood bulb out, testlight doesnt' glow bright.

The battery that's in the vehicle came with it and i have no idea how old it is. Begining to think maybe it simply won't hold enough of a charge to tolerate the drain of the computer

 

I had the same problem with my daughter's 92 Ranger. The fix was to replace the fuel pump relay that was located in the engine compartment underneath the big master fuses. It was the green relay. It only seemed to be a problem when it was cold outside. While I was fooling around with the relay (key off) I could feel it energizing and heard the fuel pump go on and off. That wasn't a normal thing with the key off. So I replaced it ($25 at the local auto parts place). Problem solved.