79 240D wont shut off with key

[Tim Finn]

I rcently became the owner of a 79 240D with 110,000 mi on it. I purchased it from the original owner.From day one I noticed that the headlights and dash lights were rather dim but I attributed that to the age of the vehicle. Recently the battery charging light would flicker and go out if I raised the idle slightly with the dash adjuster. Last night, something strange happened. After running the "rocket' for about 40 miles, I parked it and could not turn it off with the key. I used the emergency shut off under the hood and that worked. When I used the car later, it started right up but would not shut off with the key. On the way home, I noticed that the headlights would get brighter and then dim. This happened a few times. I know very little about cars and am on the island of Maui. I wondered if I should take the car to a service station, or seek the assistance of an auto electricial person (ther are 3 listed on the island). Any ideas whats happening ?
Tim