Very generally, touching is OK unless specifically not allowed, bracing is not OK unless specifically allowed. With the infant seat, if it is touching, you can have issues with the seat not clicking all the way in the base or getting your hand in to release it. For those reasons, I generally encourage parents to leave a slight gap. For a convertible seat, those don't apply, so you go back to the manufacturers' rules for the seat and the vehicle. Evenflo is the only one I know of that had rules around not even touching and required even a large gap. Some vehicles have warnings in the manuals about pressing on the front seats with cargo (usually in the 'cargo' or airbag section, not the child safety section), but it's doubtful that a 1996 vehicle would have this as it is usually tied to airbag sensors in the advanced airbags.
If you can fit something, even a piece of paper, in the gap, that is obviously not touching. Touching, I would consider either not deforming the fabric/leather at all or just barely deforming. It's hard to give a written definition - usually I run my hand between the seat and the vehicle headrest and if I can get it through without hurting myself, just smooshing the foam a little, that's "touching". I know, not very scientific, lol. Bracing would be pressing on it and not able to even squeeze something between them.
Mama to three boys ('03, '05, '07)