solenoid for electric fan

hi newbie here and i have a 2007 tacoma that i want to install a mark VIII electric fan on using a constant duty solenoid instead of relays.

What's the difference?

A solenoid is the "actuating" or coil part of a relay. A solenoid can also be a coil that moved something - eg, a core that gets pushed through - as used in door chimes, klaxons, locks etc. Solenoid is often used as a generic term like relay and maybe contactor. If by constant duty you mean full-time rated, most automotive relays will do the job. Of you want a latching relay or contactor, I would question why, and point out that you would need suitable control circuitry.