Oh wow, we looked into this at one point..... it's such a hassle, I think, the way cars are designed nowadays. It's really best, we decided, just to let the car dealer install the kind of rack with adjustable slidey bars that are what the roof pack actually attaches onto. The dealer can find out where the roof supports are hiding under the headliner, and do it right if it can be done at all. Otherwise it's as Bert said, and then there's always the tension to adjust and the interference with passenger headspace, etc.