On the head at the level of the valve cover gasket to the right of the oil filler cap as viewed from the front (gasp for air) there are a couple of bolt heads(13mm or 15mm socket). Remove one of these bolts and fill through the reservoir until coolant comes out. The manual says to open the front bolt, but it sure looks like they lead to the same chamber.

Sixto
91 300SE
81 300SD