how does the air bags come into play.

 

You know that is a good question.

I can only guess the sensors on my 2000 pick up had all ready been removed as I never saw them or even any plugs for them .

Looking in the Electrical manual the ones for the 2000 Tacoma are in the engine compartment.

Not sure about the 1999 I don`t have that year but someone should know. When ever a BFH may be used disconnect the negative battery cable.

 

Remove it or replace it or...?
Pic of the bumper mounts on a 99:

For each side a bolt above the body mount, one at the end of the crumple bar or whatever it's called. Either three bolts at the frame endcaps or possibly you might be able to unbolt the valance. There might be two more bolts at the top corners of the bumper to the fender. Other than weight and rust it shouldn't be too bad.
The airbag sensor is nearby but I don't think it comes into play with the bumper. Don't forget to unplug fog lights and blinkers before you drop the bumper.

 

Airbag sensors are not located on the bumpers. Replacing the bumpers will not affect the airbag sensor mounting, and assuming you are using OEM bumper parts, will not affect the function of the airbags. IIRC, 1999+ 4Runners do not have sensors mounted to the inner fenders like the 96-98. I believe It's all built into the computer/yaw rate sensor under the center console.

Regardless, you don't even need to disconnect the battery. You can change the bumper without any ill effects.

 

if I do disconnect the battery will it disable the air bags while I work on the front end ???

 

not necessarily, since an inadvertent charge could technically set them off. However, it is much safer to disconnect battery and wait 60 seconds or so anytime you do any serious work. That being said, I usually never bother disconnecting the battery unless I am changing it... :/

oh, and on a 99, your airbag sensors are not on the bumper, so you're fine. I cut the stock frame mounts off my '99, cut frame back ~1" and welded on new mounts, I think with the battery still connected too. I seem to remember airbag sensors mounted somewhere on body behind bumper? Pretty sure the wirewrap or wires going to them are yellow to stand out.

 

The sensors are never mounted to the bumper in any year 3rd gen 4Runner.

I have been working at a wrecked yard for 12 years, working exclusively on Toyotas, and never disconnected a battery before removing a bumper. I have even removed airbags and never had a problem even with the vehicle running.