| I would take your multimeter (you do have a multimeter, don't you?), set it to measure resistance, and test continuity through the red and yellow, red and black, and yellow and black wires. Note the readings you get from all three combinations.
If one gives you a reading > 6kOhms while the other two combinations overload, the one giving you a reading is for the signal and ground wires. The third wire that doesn't give you continuity with the coil is probably a shield ground; wire it to the back of a pot shell or, if your control cavity is shielded, to a bare spot on the copper foil or to the ring terminal that the shield ground is connected to.
In the case that all combinations give you some continuity, the pickup is a tappable humbucker, meaning you can cut out one coil and get a single-coil sound out of the pickup. In that case, black is ground, red is the "single coil" and red is the "phantom coil". If you would like to tap the coils, look for a wiring diagram for a two-pickup bass with coil tap; shouldn't be difficult to find. If you don't want a coil tap, solder both the yellow and red wires to the bridge volume pot (usually to the wiper; look at where the signal hot wire of the P pickup is wired to its volume pot). | 
