Many of the home alarm people use a magnet / reed switch sensor.  Uses no
power.  Provides either closed when closed or closed when open at the
terminals depending on whether the reedswitch is NC or NO (normally
closed/normally open)  Mount the detector on the van and the magnet on the
top.  Piece of pie.

Just wire it into the door switch circuit.  When it's open the interiour
(check that clam-boy, I'm learning English as a second language) light will
go on as if you had an open door.

Of course you could wire it to the electronag if you still have one.

FWIW Incandescent lamps (QI,Halogen,reglar ol bulbs) are insensitive buggars
that do either DC or AC just fine with no additional current draw from your
battery.

pensioner