If you happen to have a dead power supply and molex fan laying around, you can recycle them into a molex y splitter!
Disclaimer: I am not responsible for any injuries or accidents that occur from doing this. Be very careful and only do this if you know what you're doing! Do not open your power supply unit if it has been recently used, as you risk electrocution if the capacitors are still charged. You have been warned!
First, start by cutting off all of the wires coming out of the power supply - the more slack the better!
Next, you'll need a female molex plug, like the one from the fan, so cut off the wires leading to the dead fan - you wont need them anymore. Feel free to cut them closer to the plug in case you wanted to keep the fan parts for any reason. If you look inside the molex plug, you'll see 4 pins. The pins are held in place by a sort of harpoon - you can use a small screwdriver to bend the 2 finds on each pin inwards, so you can pull them out the backside.
Once you get all 4 pins out of the molex plug and have cut the 2 wires going to the fan, strip the 4 wires from the molex (or sata) power cable you wish to attach which you previously removed from your power supply. Then solder these 4 wires onto the corresponding ends of the pins that you just took out of the female molex plug. Be certain you don't mix up the two black wires! They are not the same! Also try not to get too much solder on the connection or else it wont fit back in the molex plug!
Once you are done soldering and it has hardened enough, slightly bend the 2 fins on each of the 4 pins back out a little so that you can re-insert them into the female molex plug. Finally, gently feed the 4 pins back into the molex plug in the correct order. Once again, be absolutely sure not to mix up any wires - especially the two black wires! If you are unsure, compare to one of the male molex plugs that you have attached.
Once you have finished this, you have made your own molex y splitter! I've made a couple of these now, they can be super handy, especially if you want to mix and match different types of power plugs on a single strand (ie. molex, sata, floppy, etc.). Just be careful not to add too many plugs and overload your power supply! Enjoy!
Some pictures from one of the molex y splitters I made are featured below.