VSF,
the cabin of the 189 is large enough to have four seats across the front bulkhead with then 2 seats in either corner at the rear. The tunnel access from the cabin through to the boot is then open and accessible between these two seats.
In the picture a page or two back where you see the row of four seats, simply take out the middle two for the tunnel access. The seats in the pic are standard pax seats also.
Not knowing for definite whether this is how the SAR cab will be but it's what AW did with the 139.