The 6 series, either coupe or convertible, do not have spacious back seats. The seats are for carrying small parcels or your briefcase: not people: at least not people you like.
There are 7 seats in a BMW X-5. There are the driver and passenger seat, the 3 middle seats, and two seats in the trunk. The two seats in the trunk are small and non-spacious, so they may not be considered seats for large people.
You have to remove the top part of your right rear passenger seats the battery is located under.
The 2003 BMW 3-Series has 54.2 in. of rear shoulder room.
The 2006 BMW 3-Series has 55.1 in. of rear shoulder room.
The 2009 BMW 3-Series has 55.1 in. of rear shoulder room.
The 2014 BMW 5-Series has 38.3 in. of rear head room.
The 2014 BMW 7-Series has 57.4 in. of rear shoulder room.
The 2014 BMW 6-Series has 49.7 in. of rear shoulder room.
The 2013 BMW 1-Series has 53.0 in. of rear shoulder room.
The 2005 BMW 6-Series has 28.8 in. of rear leg room.
The 2002 BMW 5-Series has 37.8 in. of rear head room.
The 2014 BMW 7-Series has 38.5 in. of rear head room.