Try the Facebook group Car Seats For The Littles. They'll know. I wouldn't imagine it would be a problem as long as you use the angle adjuster (baby has to have full head control).
I have as Volvo XC 70, and the Diono didn't fit rear-facing. It may have with the special wedge you can buy for an extra $20, but that seat is just freaking HUGE. We love it forward-facing, though.
I have a VolvoV50, so the backseat is pretty comparable to the S40.
I tried the Recaro Performance Ride which was recommended for smaller backseats and HATED it for rear facing. The associate at Buy Buy Baby didn't recommend the Radian because of the angle on the rear facing. I switched to the Chicco Next Fit and LOVE it. It fits really well rear facing.
Re: Volvo s40 and a Diono Radian Help!
Baby Boy due October 2017
I tried the Recaro Performance Ride which was recommended for smaller backseats and HATED it for rear facing. The associate at Buy Buy Baby didn't recommend the Radian because of the angle on the rear facing. I switched to the Chicco Next Fit and LOVE it. It fits really well rear facing.
Good luck!