Problem with a Navigator

I have a 2002 Lincoln Navigator V8 5.7 I had to replace the Motor already last year. My problem now or probably always has been that out of no where it doesn't want to turn on. Before it was when we were cleaning it and doors were left open for about 30 minutes, that was enough for not turning on. Later the problem was that even if nothing is on inside it doesn't turn on. then two days later out of no where it would turn on again. Now it doesn't turn on at all unless u jump start it, doors have to be closed in order to recharge other ways u can spend whole day charging. We've replaced battery several times, since they said it needed a bigger one for the added tv. which is not working now.!! (no signal shows as if it is on, it broke)We went to O'reillys, they said it was the alternator, we went to mechanic and he said it wasn't that, that it could be an electrical shortage, what is that? It doesn't turn off while running, it makes clicking sounds when trying to turn on, light and radio work even if it won't start. But it also makes a sound when parked and off, the manual says its the tire airsuspension!! What can be wrong??? o it has also started to hesitate at a certain mph about between 50 to 60 and theft signal blinks, rpm doesn't go down nor battery signal? could this be same reason? transmission? we've changed fuel filter........didn't do it for about 2 weeks started again. Truck has 140,000 miles.

 

It could be your alarm system that is preventing you to keep it running or draining the battery.

But, if the alarm was the issue, jumpstarting the vehicle would not work either.

I suspect a bad alternator. A short in the alternator will prevent your battery from charging and other vehicle electrical problems.

How did this so called mechanic test the alternator?