also quite likely to be the connection to the solanoid under the car...

you can reach it without jacking...

with a cold engine and the engine in the on position but NOT started (i.e. all the idiot lights on but not running!)

put car in 4th gear and punch OD button...

does the OD (5) light light up on the dash..if the answer is YES then you know the switch is ok, and the relay is a good chance of being ok 9could still be U/S - but not likely)

reach under and pull the connector (spade connector), then touch it to the spade..does the solanoid click? if YES then you know the solanoid is ok and the relay and all the electrics, if no then yeither the solanoid is U/S or there is no juice getting to that connector (on the wire)...test the connector using a 12 volt bulb wired to the earth on the car and the spade connector - if it lights u got juice and the solanoids u/s...

now one slight variation here (ask me how i know....) is the intermittent defective wire, if its this if you reconnect the spade and jiggle it around (the wire that is...) you should heare the relay click in and out..mine was intermitant and had corrosion in the joint between the cable core and the spade connector....

also you can try SOAKING the under side of the car with a hose...mine would work until it dried out - again a sign of corrosion issues..