Will I be able to hear my subwoofer?

I drive a 2011 ford fusion and i have just laying around a pioneer shallow mount 12 inch subwoofer that peaks @ 1000watts max. If I run a 1000watt amp will i be able to hear it in my trunk? Keep in mind my backseat does not fold down.

Peak is not the measure you go by. Maybe the sub is 300 rms watts. Look up the model number to be sure. You can destroy a speaker feeding it too much power. Match your amp to the speaker by equal or less RMS watts, and equal or lower stability than the speaker's impedance. The speaker needs a cabinet. Yes, you'll be able to hear it, unless your muffler is missing.

The Pioneer Shallow Mount subs are 400W RMS - 1,500W peak so theoretically yes you could put a 1Kw amp on the sub but you will come close to it's max rating, so a 700W-900W amp would be more fitting for the amp.
And one single 12in will give you good boom, but nothing you can shake the car next to you with unless you got a 5Kw RMS sub being pushed hard!