Testing circuits

I'll say with the battery relocated to the boot if you pay someone else its going to go one of two ways.
1. They cobble their own method. This is most likely. I see electricians do this in residences all too frequently. It's faster and more profitable.
2. They spend the time to figure out what was done, whether it makes sense, and then comes up with a plan. You're going to have to pay them for their time.

In either case if you haven't learned what the car has now, you'll have little to judge whether the mechanic is doing right by you or just making a hack job that will bite you down the road.