Vibrating seat
 

I noticed that when I am driving my car at above 50mph, the passenger seat vibrates constantly (of course, when no one is sitting in it).
Also, the steering vibrates quite a bit when I brake and I feel vibrations or pulses in the brake pedal. I believe it is due to rotors, but which ones? Also, is the vibrating seat due to these rotors ?
Thanks

first thing i would chk wheel balance ( all 4 ) for the vibration on seats assuming seat tracks bolts are not loose or tacks are worn,,
As for the vibration when braking , go to the DIY section there is a write up on how to detect it ,, most likely you got front rotors warped and depending on the thickness they can be machined or if below specs will need to be replaced ,,