give us an idea ... what year truck you have ? what engine does it have ? how many miles on it ? what about the heat don’t work ? fan blow ? blend door move ?

get the motor up to operating temp.. pop the hood and feel the 2 small heater hoses going into the firewall... are they both warm or hot...

is your heater control working properly.. it switches from hot to cold.. no sure if 99 has a blend door. if both hoses are getting warm .. that tell’s you , you have flow through the heater core.. you have no heat whatsoever in the cab ?