I have better luck if you connect into the middle of a wire instead of at the end.
Yup, this is a likely candidate for what causes the problems. These taps have two trays, one for the wire you're tapping (in this case my bikes brake light wires) and one for the accessory wire. The accessory wire tray has a barrier on one side so you can slide it into the tray only so far before it hits the barrier. Two potential problems. First, the actual wire part of the accessory wire may not extend to the end of the wire casing (that can be visually inspected and cut back if necessary) and Second, the plastic barrier may prevent the accessory wire from sliding into the tray far enough that the metal tap grabs it properly. The way to deal with that is cut the barrier out of the tray and slide the wire further through so it grabs the middle as Jamie suggests.
Usually these types of wire taps come with the accessories one's installing. They're kinda cheaply made imo...and priced accordingly...so that's what the accessory manufacturer tosses in the box. Another tap you might find with some accessories (I know Denali and Equinox has these in their Light Kits 'cuz I looked) or purchase separately are Posi-Taps. Another simple tap, but maybe cost eighty cents instead of twenty cents, and definitely work better.
