Here's a link to a British G-Wagon forum that may be able to give more information:
http://p15116006.pureserver.info/~admin19/forum/index.php
I know nothing about your engine. But when the oil pressure sending switch failed in my 409d, it failed in the 'off' position. In other words, the light did not work at all. I do not know if this is the only way that pressure switches fail.
You can check the pressure a couple of other ways. Look in the valve cover thru the oil fill hole and see if oil is getting to the valve train. You could also loosen the cover on the oil filter housing and see if oil squirts out (this would be very messy if you have oil pressure). But if the filter housing filled with oil, reducing the oil level, you must have oil pressure of some kind.
T'ing into the oil cooler hoses will not help much, because if it is like the system on the 617 engines, the cooler is thermostatically controlled with no oil going thru it at low temperatures
Overall, I'm inclined to think that sticking the pin into the hole in the sender unit damaged it. If I were you, I'd bite the bullet and replace the sending unit with a new one.