Cut a strip of wallpaper to fit from the ceiling down to the floor or wood molding.
Apply wallpaper paste to the back of the wallpaper just far enough to hang the wallpaper from the ceiling to the top of the radiator.
Measure the distance from the floor the any wall brackets on the back of the radiator and from the side of the wallpaper strip to the bracket.
Mark that point with a pencil on the back of the wallpaper.
Cut a slit from the bottom of the wallpaper up to the pencil mark on the back of the wallpaper. For the radiator pipe, measure and cut a hole connected to the bottom by a slit so the paper goes around the pipe.
Apply wallpaper paste to the rest of the wallpaper strip. Allow it to drop into place.
Run a broomstick behind the radiator to press the wallpaper into place on the wall.
Paint the rest of the room as you typically would, painting as much as you can behind the radiator with a paintbrush or roller.
Attach a paint roller to the end of a long handle.
Dip the paint roller in paint and allow any excess to drip off.
Apply the paint to the wall behind the radiator taking care not to touch the radiator with the paint roller.