The positive (+) one.

Then he has the joy of chasing down and disconnecting each item in that
circuit. Bulbs are easy, just unplug them. Other items may be more difficult.
Good luck.

I marched for peace, right along with the rest of the Division.

Either one will do the job, since it just completes a series circuit.
Tell him to use a LOW WATTAGE bulb, like a side marker or dash light bulb
if possible. It's "possible" the circuit doesn't draw enough current to
light a higer wattage lamp, but still enough to discharge the battery
given enough time.

Then, where does he go?
Isolating the circuit is the first thing. When that's done, you have to
figure out what specific componant is the culprit.

Somethings to check-
Possible trunk or glove box light not going out when it should. If he
can't get to checking the circuit immediately, he can at least temporarily
remove those bulbs. They tend to be a very common culprit, since they are
out of sight, out of mind!
Does he have any electrical componants that no longer work?
Example- I had a friend with a 70's era Chevy that had a bad clock (the
old analog kind). It had quitworking, but was drawing enough current to
discharge the battery overnight.
Another caveat- When he is checking the interior light circuit, make sure
he has the door closed;-)

I'd be willing to bet it's some sort of sticking relay.  I'd lean away from
a small wattage bulb like a glove box light since I've left mine open for
two days before...it wasn't drawing enough current to drain the battery;
the starter whizzed over as normal (turning over a 400 Pontiac at that).

I've seen several pop-up headlight motors flake out & run batteries down
before.  I'd do the test-light thing while removing fuses...you'll find
the prob.

Good luck,
Joel

I'd pull each fuse out, one at a time, and connect an amp meter (or a
multimeter set on the 'amp' range) across the terminals of that fuse. Do
that with all electrical devices, such as interior light, wipers, brake
lights, radio, etc. etc. switched off. If the amp meter shows a current
between any of the fuse terminals, then I'd try to find out what device that
particular fuse is for, if it is supposed to be drawing current, and how
much current it is supposed to be drawing. Some devices such as a clock may
draw a few milliamps of current, but anything like an amp or more is very
suss.

Henry.

Go buy two small male plugs, then jump the culprit fuse terminal to an
equal amp accessories fuse terminal.   (Another words, rig so that
circuit is open when key is out, temporarily.)