I had the same problem on my 2000 GMC Sierra (v8) and what i did was removed
the two hoses that go to the firewall down to the heater core, and placed my
garden hose (cut it and clamped it) to the left outlet on the heater core
(reverse flush) and clamped about 6 foot of hose to the right inlet, just so
that all the water drained away from the motor.....then i cranked the water
hose, and flushed out the core!  at first the flow of the water was weak,
but after a minute or so it flowed strong!  unblocking the restriction. let
that flow for about ten minutes, then switched the hoses and flushed it in
the normal flow direction!  now my heat is HOT!  just pop the hood after
your at normal operating temperture and feel the hose that runs to the inlet
(right side - firewall) should be hot, as well as the left side (Outlet) if
the left side hose is cool,  you may have some heater core blockage!