Electric Fans are high current devices, You should Always use a relay when installing one to keep from frying a harness.
If you just want it in "Run" all the time the best way to do it is get a relay that will handle the draw of the fan , whatever that might be, I think usual is 30 amps.
Locate and wire the Coil for the relay, one wire to a hard ground, and the other to about a 2 amp fuse, then to any source that is hot when the key is on. This is your control circuit.
Locate the Normally open Contact and the Center wiper of the relay, wire the center wiper to the Fan motor, and the normally open side to a properly rated fuse for the motor, then to any Battery source, or the harness side of the fusible link.
That's it. You will have a fan that runs when the key is "On" at all times.
Are you sure you don't want to temp control it?
In some cases running a fan continually, can cause over-cooling which is as bad for the engine as under-cooling...
You can add a manual switch by either adding a switch between the relay fuse and the switched 12 volt source to manually control the temps. Or between ground and the ground side of the relay coil...But this too has pitfalls, If you forget to turn it back on...you can overheat.
Hope it helps!