how can i find out why power is draining from my 1995 ford escortTD?

i own a 1995 ford escort turbo diesel which has been running great for a year untill about a mth ago. i came to start it for work 1 morning and the battery was flat. bumped started it and went to work but when i finished work, it was flat again. after jump starting the car, i was on the way home and noticed the rev clock wasnt working and my battery light was dim. so put a 2nd hand battery on and it was fine 4 a while then was same again. suspected alternator but bought 1 from scrap yard and it was same.tried another battery, but same. the power is draining somewhere but is only lost when running. if any one has any clues, giz one please coz im that close to just seeing how much i can get for selling, but its been faithful and i dont want to giv up, also how can i test the new alternator is working? thanks; matty.