That's because the car does not have a front seat (from the game logic standpoint). The bench in this mod is listed as rear bench, not the front bench, even though it is positioned in the front.
This is an issue, the mod author needs to address.

Yet, you still can drive it. But, instead of trying to enter the car, you can open the map, then select on it a track, you want to drive at and then the car. In this way you can drive the F-100 from the cab view.
But, yes, you can not sit in it while in the garage. Thia ia a bug of the mod.