1- The HORN fuse is located in the fuse panel on the driver's side under the engine hood. Of course, if it is "blown" replace with the SAME size or smaller and retest the horn.
2- Verify the horn itself (isolated) using one wire attached to the Main Battery + (POSITIVE) to the spade terminal on the horn. The horn can be accessed after carefully unclipping the fasteners connecting the front grill to the radiator area- just be careful- work slowly. The horn is naturally grounded to the vehicle frame.
3- If the horn sounds with the direct bypass voltage test move to the cab. when you depress the combo horn pad/airbag listen carefully for the relay behind the left under-dash area. If you hear it, then the relay is "closing" but it still may be bad or the voltage from the horn relay is not getting to the horn. I would replace the relay. However, there may still be a wiring problem to the horn.
4- The Horn Relay for the '00 Toyota Tacoma V6 4WD I worked on was BEHIND the cabin fuse panel on the far left driver's side (oh thanks guys). I went to the trouble of pulling down this relay/fuse block- it was not exactly "easy" but I am not a beginner and had the tools and my curiosity and sleuthness prevailed. If the new relay does not fix this issue my next step is to replace the wire from that relay to the horn. Again, not easy but not impossible either. Take you time, use the right tools and materials & check your work. The only other option was mentioned earlier- drop in a new momentary switch (lighted?) into the dash and run a new wire. I would NOT use switched power so it can be used without the vehicle running... I think it is safer. This is a view of the interior panel dropped down. One of the relays can be seen- the grey cube with a empty relay socket just to the right. Why they put these here- is anyone's guess.