I don't think that having the car inside a garage makes any significant difference to the touch voltage risk, unless the garage floor is a sufficiently good insulator.
It's not normally much bother to install an earth electrode, maybe 20 mins or so to install and test, and the connection box at the top of the rod can be buried flush with the ground usually. My garage is detached and has a TT earthing arrangement, just because I didn't want to export the earth down ~30m of cable. I drove the rod down through the garage floor, just drilled a hole in the slab and then banged the rod into the ground beneath. I did this right near the wall where the garage CU is fitted, so there's a small box at the top of the rod and the cable goes up to the CU above.
