I agree upon using a multi meter for nearly everything. sure a test light is extremely convenient, but it draws current and can cause problems if hooked to the wrong wire. it's a matter of fact that testing wires using a multi meter will cause less or no problems compared to a test light, so it's better for those who aren't pros. I did a grand am a while ago it I swear the entire car had only like 4 colors of wires. every bundle had like five wires of the same gauge and color. tap into that with a test light? hell no, it's got data wires to open the doors. now on my honda there were very few duplicate wires, so I was pretty sure what wires were what when there's only one burgundy/fuschia in the bundle.
we wired up a push button in my buddie's doors, he had solenoids already in there to pop the latch, but the buttons were under his seat or via remote, which was less convenient than on the doors themselves. so we cut a wire leading to the door to use, it was for factory power windows which he doesn't have. passenger door went no prob, would have been a bitch to go through the 2 sets of plastic plugs (gotta take the door off) and rubber boot to get to the door, so we cut one unused factory wire. the driver door however, we kinda rushed. it was getting late, there was an unused blue wire in the door, and it had continuity to a blue wire in the car, but to the side of the wire leading to the back of the car. so we hooked it up to that wire and it worked great. a few days later he calls me and tells me when he changes his stereo from cd to radio, his door pops open on the highway. every time. so we tap the wire into another blue wire in the same bundle (didn't see it the first time!) and it works properly this time. after the fact he mentions that his power antenna wasn't working earlier either! I was just glad we didn't damage anything, I'm sure the radio wasn't expecting to get a ground on its power antenna wire.
only after the wire has been verified to the best of my ability (wire matches diagram, voltage readings are as expected) would I apply voltage to check the circuit such as door unlock and such. cigarette plugs make good power sources, multi meter leads can plug into the factory plug most times.
again everybody has their own ways of doing things, in fact I bet the only thing everyone can agree on is nobody wants to fry electronics and have to replace them. if you want to be the safest and not blow up peoples' electronics, disconnect the negative battery terminal, use a multi meter, and leave your fuses out till they're needed. I also leave the horn/siren for last so I don't annoy all the neighbors.