Another twist on unik's method: (if you have electric door locks (thanks, Vic))
(a) enter rear (or sliding) door.
(b) close that door.
(c) push its lock button down.
(d) all doors will lock.
(e) pull its lock button back up.
(f) only that door will unlock.
(g) open and exit via that door.
(h) push its lock button down
(i) close that door.
(j) try to convince dog that you really haven't lost your marbles...
--dick (still working on (i))
p.s. (a) could be "enter any door", but (c) onward has to be in the cargo area.