We are flying to visit my inlaws for two weeks and will be using their 1999 Ford Explorer. Our just turned 4 year old is 40" and 32 lbs and rides FF. We have a Diono Radian and a Chicco Nextfit. We have traveled plenty with both seats. I have also thought of getting a light weight seat to use for travel. This trip has three flights each way so lots of getting on and off planes. I have seen mention of there being no top tether in the 99 Explorer. Is this true? Don't most convertible car seat require a tether in FF mode? What should I use? Thanks!