I need help to wire an in-line duct fan to my frigidaire furnace and can't seem to find what wires control what. My Technition told me I should piggyback to the blower wire. Please help.

My furnace blower has a half dozen or more leads to it all in the same plug. Which ones are hot when the heat is on? Who knows? You could check with a voltage detector, and then splice into one that was hot. You might even try to fine one that is hot in both heating and cooling.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.