On my 92 f150, I've had the same problem twice.  The air temp is regulated
by a baffle beside the heater core which controls how much air passes
through the core and and how much bypasses, thereby controling the temp.
Somehow, I have twice had pens (kids no doubt) drop through the deforst on
the passnegers side.  They go strait down to this baffle, causing it to not
close all the way.  When it is open, it is sucking outside air and bypassing
the heater core, hence the air is cold.  If you take off the cover under
your dash that covers the heater core, the baffle will be to the drivers
side of it.  Make sure when you turn your temp control to hot it closes
tightly, being perpendicular to the firewall. Hope this helps.  This is all
assumig a '97 uses the same theory as the 92-96 styre.