The PICK-UP screws into the bottom of the mag, but like Mark said it has 2 leads coming out of it with spade lugs on them. The last time I had a problem with mine the wire broke off at the sending unit. I was able to repair it (I won't go into that) and made it work for a while until I could find one.The best thing to do has been suggested, look for breaks in the wires, disconnect the spade lugs and put an ohm meter across them to see if the pick-up is open, while you have the ohm meter on the wires, wiggle them ever-so-slightly to see if you looses continuity, mine was broken inside the insulation. If everything checks out the next step would be to swap the wires on the back of the tach to see if the problem moves from one to the other.
If it turns out to be a bad pick-up and you can't find a replacement right-away I could loan you my old one for a while, it looks like CRAP but it works just fine.
Fred N358 337B 0737