It certainly sounds like your inhibitor switch is the problem. I've never had this on my Montero but I have had the same problem on a Toyo. It looks like the Reverse contact is the first to go, maybe because you always have to pass through R to get to the other gears. I think your switch is located on the outside of the trans?? I also don't think there is any way you can disassemble and clean it. What I did is check my adjustments and try to postion the switch to pick-up the reverse contacts. Having a helper in the vehicle to move the shifter and observe the lights makes it way easier. So far my adjustments have worked. These switches are expensive to buy new so getting more miles from it makes sense.