If you look on the compressor its self, there should be 2 wires, atleast on the earlier troops and rodeos with the 2.8s and 3.1s, one should be a power, and the other a ground. If I remember right, those wires were only 12 guage or so, not exactly heavy duty. A 25 amp switch should be fine. A lot of the newer systems are the same way. If the compressor doesnt have an actual plug, then they have a hot wire coming to the clutch, and a ground wire on the body. I would find it much easier to go through wires under the engine compartment then ripping the dash apart to find them. If you want to wire a relay in, go for it, but then you are running more wires and have to buy the relay. Not to mention you then have to find a 12 volt source to tap into, and then ground the relay and switch if you go with a lighted switch. If you want to use a relay, heres where to plug each wire on the relay using a common 5 pin auto relay:
87 - wire going to spiced compressor power wire
30 - wire comming from splice compressor power wire
85 - ground
86 - to load terminal on switch
87a - not used

For the 3 prong lighted switch:
power - to a 12 volt power source (fuse block?)
Load - to terminal 86 on relay
ground - to a good ground source.

You can go this route, but I dont think its completely necessary. Actually its probably a bit overkill. I did something similar to this to disable the TOD system on my explorer. I wanted an actual 2wd vehicle and not a permanent AWD with the ability to run 4 hi and 4 lo. Now I have awd, 4 hi, 4 lo, 2 hi and 2 lo. But I still believe you can just put a 30 amp switch inline and be fine. The lighted switch will be nice because you will know when the ac is on or off.

Dan