If you really wanted a quick fix, you could run a new relay and fuse to the battery and trigger the relay with a switch. This all assumes that the fan is actually working though. You would be much better off just troubleshooting what you have already and finding out where the problem lies. The fan circuit of most cars is usually very simple. Either the fan is controlled by a coolant temp switch, or the fan is controlled by the ECU. Either way, that control signal turns on a relay which switches the (higher current) power to the fan. There is a fuse for the fan power, usually before the relay.

It's easy to check where you have power (or don't), and if you have some voltage drop problem caused by a bad ground or corrosion. Whatever problem you have it should be fairly easy to spot based on the very visible symptoms you described.