The warning chime is generated by the GEM and is not easily defeated.
You might be able to disable it for the "door ajar" warning by cutting a few wires going into the GEM, if your brave enough to do it.
The 99 model year wiring to the door ajar switches are as follows:
Left front; yellow with a black stripe
right front; gray with a red stripe
right rear; pink with a light blue stripe
left rear; light gray with a yellow stripe or a pink wire with a light blue stripe depending on what "power group" your truck has.
Pin numbers 5 6 7 and 8 leading into the GEM.
Do so at your own risk.
While I have the wiring diagram book for the 99 model year, this is how I interpret it.
Your results may vary and if you screw it all up, I do not want to hear about it.
Might be easier to just get used to the noise