easy noob question

i'm mad i can't remember the answer to this. but here we go - do i want the constant or switched power wire when hooking up a head unit? there is no pigtail (previous owner snipped the factory pigtail). there's only one power wire comin from the head unit. i was gonna hook up both constant and switched but thought it may be a bad idea. any suggestions? sorry for the noob question

edit: one more question. my friend has a free air set up. it's a little hard to explain but i'll try my best. he bought a rear seat delete kit (basically it eliminates the back seat so it's all flat) he wants to mount the amp INSIDE of the rear seat delete where the baskets of the subs are. he's worried there will be too much vibration from the subs that it will damage the amp. any suggestions here? i think he is goin for a stock look with safety in mind. i haven't dealt with free air apps. to know enough about this stuff.

switched, provided it gives enough power for the head unit (~10 amps). that way it comes on when you turn the car on, otherwise it would always be on. but you also should have a constant (less than 1/2 amp) to keep the head units memory.
as for the second question, i'm not sure. but if the subs do cause much vibration under the rear seat delete i would not reccomend it.

Yep, you need both a const 12V AND a switched 12V ... but not connected to the same place.

I will sometimes hook up a head unit with constant power to both the memory and 12v switch wires. It doesn't hurt if the unit will power off completely or if it has a detachable face. I have my reasons why I like to do this, but you may or may not. I'm just noting that you have a choice. Just don't leave it on too long and kill your battery.