> The rad's only about 14 month's old --- I'd hate to think it's hosed
> so soon. Is there any way of testing it?
> 

Radiator ought to be fine, unless you've been dumping in stop-leak or
something. Time to sit back and think about the possible points of
failure. Cooling as all about water flow. If the water doesn't flow,
it doesn't get cooled fast enough. Water pump is worth casting  a
suspicious eye upon, since your belt broke. I don't know what
condition the belt was in, but it seems like an awfully interesting
coincidence. If the pump's dragging, not spinning fast enough due to a
failing bearing, that would certainly reduce your water flow and could
put enough strain on the belt to kill it. Your pump must be at least
partly functional, though, as the addition of the heater  brought the
temperature down. If the belt hadn't failed, my suspicion would fall
first on the thermostat.  A bad thermostat will block flow through the
radiator, but not the heater core (which is independent of the
radiator circuit). I'm bettin' the fan switch and radiator are fine
and the water just isn't getting up there. Might be worth trying a new
thermostat first, as it's easier to do than change the water pump.