99 Ford F-150 won't start loses power when engaging.

WTF ? you have flying sparks, a shop attempting to repair? But dont have an estimate for complete, proper repair? You describe battery cable and fusible links, as well as power distribution cable related issues. the cable ends must be able to carry the current, be clean and tight, not full of resistance. Back track to that, and lets not take any parts off youir list just yet.. have the inspections, prechecks, circuit protections, and the battery itself, its service, its being filled, cleaned, and passing tests.? does that somehow pass as good to use? I am next going to suggest inspection of the starter relay. And see if the tech who "worked" on it, has any suggestions from what he learned or noticed.

A 12v test light is 4 bucks, you can check power at each point leaving the battery. a battery brush, baking soda, volt meter, wrench, screwdriver. it can all be rounded up for 10 bucks nowadays. cable ends are like 2 bucks each, see where this is going? Battery tests are free at places trying to sell you a new one. A skilled tech and his equipment costs more, tests are run, maintenance and parts replaced to achieve basic function, lets say. then, tests that prove you need a battery, relay, cables, or their ends, or have some part or problem that would cause arcing. I would expect an answer in the first half hour.

Sparks flying indicates a short circuit. A visual check should show burnt wires or components. Trace the positive wire from the battery to the location where you saw sparks flying and look for damage.

Right, but it does so during cranking only, the sparks are by the right tire under the hood, and the problem has discharged the battery, they hear the starter relay. trying to connect with low voltage. The more I reread this, the more I think the shop needs to look again. What operates only during cranking, is "not connected" when the switch is off, even though the positive cable is always connected to the relay. they "state" that cables and starter have already been checked, and they are ok? You hear the relay flutter, but on these trucks you cannot see it. The cables that were "checked" to be OK, go to and from it. It is covered by a sight shield and no longer mounted at the fender, But around the battery at the rear of the engine compartment. The vehicle was fine/ then it wasn't. Then checked by a shop, then they asked us, did it drive home? or is it sitting with a dead battery (causing flutter sound) outside the shop? Or is there unmentioned information? BECAUSE THESE STATEMENTS DO NOT ADD UP. The battery, relay starter, and cables are all at the same point viewing it from the side to see sparks, the battery is discharged if the relay flutters, and the burning smell indicates a fire occurring ( but only when attempting to crank) This system needs step by step disassembly, tests, inspections and a correct diag, not to be returned to the customer. Now they have to go step by step telling the rest, or check every part for improper assembly, sticking, shorting, burning, poorly connected, etc. It is important because they are going to lose the rig in a fire, and insurance of the shop or the vehicle owner may not save them. They need to find it, tell the rest, or, disconnect and tow it back there. It just seems their information has a pretty large hole in the story, Just sayin.