I assume you have a RHD Defender, so your AC's blower/fan is on the left, it sits with the evaporator there. So all the vents on that side are closest to the fan and will have the most air power b/c of their proximity, those on the right will have the least. B/c of the placement of the fan and evaporator so close to the floor, they both can get really dirty with lint, dust, dirt, etc. Probably the worst location to have a fan/evaporator but a Series/90/110/Defender wasn't really made for AC in the first place, so the fact its even there is somewhat of an oddity to begin with. I say this having an '03 with a AC system that I'm slowly reconstructing from missing parts and creative repairs. It's pretty easy to pull the whole blower/switch assembly off the dash and get inside and clean up whatever may lurk in there.