You need to be careful for several reasons.
First, the voltages can be VERY high, in the order of 10 to 15 kilovolts. The backing plate will not take this sort of stress. The voltage rating of the coax may be exceeded, and the loss through the coax will be excessive.
If you with to use a coupler, it should be mounted as close to the base of the antenna as possible, and all of the leads must be short! Read that as inches, not feet! And it can't be coax.
While using an auto coupler has advantages (rapid QSY, and no tune up worries if done correctly), you'd be better off with resonant antennas. You have to remember, an HF mobile antenna is a compromise among compromises; there are better alternatives when stationary operation is the objective.