Yet another ..........
Well this is how I did it !
FWIW
As a proud owner since three days I almost made my neighbours Great grandpa his Late grandpa (gee how fast that old fart could move :-) )
Went down to my FLAPS and bought a "back up - alert " kit probably made in Taiwan .....
It did fit right into the socket of the standard BU light is equipped with a 21 W halogen bulb and automatically sounds when in reverse. Beep-Beep - just like the cartoon "Roadrunner" - how about that Coyote ;-)
Works OK, didn�t improve MY rear vision much though......
So I went back to the store and bought a square WORK LIGHT  just like the ones they use on excavators and timberjacks, with a 55 Watt halogen bulb, that I hooked right onto the cable to the standard BU light in order not to forget to turn it off - or search for the swich in the dark when I need it !!
No problems with fuses nor bulbs in a year and a half, I did put it on the rear bumper next to the service door but my guess is it would be even more efficient mounted on the roof where it could - wired to a swich - be used as "loading light" for your trailer too.
But then you have to pull a mile of wire to get to the dashboard .......

Calle
-88 Vanagon
Mullhyttan, Sweden