my 1999 navigator takes too long to start blowing hot air what coul be the problem

It will take a few minutes after your truck warms up. If it isnt working when its warmed up it could be a thermostat sticking.

Ford products are notoriously slow in warming up. That goes back for as long as l can remember. l had a '51 Merc and a '57 Fairlane which took forever to warm up. My '68 Mustang and 01 Ranger are the same way. The Gm is I've owned from time to time seemed to warm faster.

it might need a new thermostat or the heater core could be going bad in it

if it gets hot when it does warm up its probably the thermostat. If u care to check a few things u could go under the hood & check how warm the radiator hoses r & the heater hoses u could find where its not circulating good at to pin point the problem. Also check ur coolant level low coolant will stop u from getting heat & depending on ur driving it might be that once u r driving it starts to circulate where when at idle or slow driving its not circulating enough.