I have a 1999 f150 xtl and i am trying to find what the frequency range is on the stock door speakers.

 

Hey nash.
I can't answer specifics for frequency ranges on stock speakers for a 1999 xlt?
But after having my 2003 f150 xlt for 10 yrs, I will say...
Stock speakers are garbage.
Not sure I ever thought about frequencies for those speakers
anyways becasue I knew they were garbage.
(nothing against what you're doing, just my personal opinion)
Any quality speaker you install after listening to the stocks will sound awesome.
Providing clean power will take the awesomeness even farther.
If possible, hook up an amp to those speakers for the highs,
and hook up / run a sub for the lows.
Then it all works out nicely.
Good luck.

 

run a 2 channel amp to your door speakers, tune the cross over on the amp with a cc1 and then you know where they will play at. they will sound a lot better then stock or even replacements running off the headunit.