Vibration in your seat predominantly means that it's the rear tires
that are at at fault; and the slight vibration at the steering wheel means that there is a little bit of the front as well.
Now that we know the rear tires
are the BIG issue, lets start with some things. First of all what rims are on the car? Are they BMW? What size are they?
Next the tires
, What tires
do you have and what size are they? The tires
can be a HUGE issue! Where were they installed and how were they balanced? I suggest a tire place use a Road Force Balancer which warms up the tire and applies a load to it as they are much more accurate.
Third, do you have any sort of spacers, adapters or foreign objects between the wheel and the hub of the car?
Fourth, What is the condition and torque on the rear lug bolts? If improperly torqued or if damaged, the lugs cannot do their job properly!
Fifth, how are the bushings on the rear suspension?