Stock rear bench is unsafe
My Mitsubishi L300 4WD is built in 2001. I couldn’t believe that in that year it still was legal to sell cars without any seat belts on the rear bench. The front seats are equipped with airbags but apparently a wooden plate with 1 cm foam was deemed acceptable protection for the rear passengers. The stock rear bench is constructed out of fiberglass which also formed the cabin wall. I decided this rear bench and cabin wall needed to go. The cabin wall is screwed and glued into place. I had to “hulk” it out of the van.
More safety and flexibility
I bought two rear seats out of a Renault Espace including mounting rails. I believe they are out of a 2014 car. Safety-wise they offer integrated 3-point seat belts and isofix to mount a childs seat. They also add a lot of flexibility. You can slide them individually from front to back, turn then around, fold them down to form a table or remove them completely.
Building a mounting frame
The seats came with the stock mounting rails. The bottom of the rail is formed by a steel plate, approximately 8mm thick. If you manage to mount them in a structurally reliably way, the resulting installation will be safe. In the construction and mounting of the frame only steel 2,5 mm up to 5 mm thick is used. I used M12 bolts to mount the frame except for two stock M10 bolts that are used to tie down cargo. When bolting the frames to the body, mostly stock mounting holes where used. Only two additional holes are drilled. I’ve numbered the holes in the pictures 1 through 4, front to back.
The front frame is a box beam 6 cm x 3 cm, 2,5 mm thick. The rails are mounted through this beam with M12 bolts. The two outer bolts are running through a stock hole (1) in the body, strengthened with a 4 mm mounting plate underneath. I assume these holes are used to mount a bench in vans that come with seat belts on the rear bench.
The rear frame is also a box beam 6 cm x 3 cm, 2,5 mm thick. The rails again are mounted through this beam with M12 bolts. Two additional M12 bolts are running through newly drilled holes (2) in the body. Two square pieces of steel, 4 mm thick are welded to the beam so stock holes (3) can be used with M12 bolts, strengthened with a 4 mm mounting plate underneath. I assume these holes are used to mount a seat belt subframe in vans that come with seat belts on the rear bench. Additionally. two bars of 5 mm steel are welded to the beam to reach the stock cargo tiedown points (4). The cargo area has 4 tie down points. This way, two of those are used securing the rear frame. The stock M10 bolts are used to mount the steel rods to the body.