It is possible to use a tow bar mounted bike carrier on a vehicle with a rear door mounted spare wheel but you need to go for a rack that can accommodate the spare wheel.
Thule have a couple of tow bar racks where you can get an adapter for a spare wheel. This adapter replaces the standard upright with one that has an offset to accommodate a spare wheel. The spare wheel takes the space of the bike closest to the back of the vehicle, e.g. you would need a rack with three-bike capacity in order to take two bikes on a vehicle with a rear door mounted spare wheel.
The Thule VeloSpace 939 could be an option. You can carry two bikes on a vehicle with a spare wheel with the VeloSpace XT 3 bike spare wheel adapter no. 939400, or three with the extra bike adapter as well.
The Thule VeloCompact 3 bike tilting carrier no. 926 could be another option. You can carry two bikes on a vehicle with a spare wheel with the ‘Thule VeloCompact spare wheel adapter no. 9241’, or three with the extra bike adapter as well. Unfortunately this bike carrier does not fit the 2020 onwards model of Landrover Defender.
Some BuzzRack bike carriers (including the Eazzy,Racer and E-Hornet range) may also be an option as the upright is positioned between bikes one and two on these racks, so it doesn’t always clash with the spare wheel. If you are interested in any of the BuzzRack bike carriers just contact us and we can check this for you. We would need to know the clearance between the center-point of your tow ball and the edge of the spare wheel (just measured horizontally from the top of the tow ball) so we can check this for you. A couple of photos that show the tow bar and spare wheel would also be useful.