2000 dodge durango won't turn over

just bought a 2000 dodge durango and on the way
home with it heard a ticking sound and all of a
sudden the car died on me I was told it was the
starter so I replaced it on side of road with brand
new one still won't start noticed the battery amps
were going low so I had the alternator tested and
passed three times don't know what else to do
battery isn't even six months old even put another
battery on and still won't turn over any one might
have an idea as to what's the cause of my durango
not turning over to start also checked all fuses and
put gas also spark plug cables and spark plugs still
don't know what is up.