Why would you want to bother to add an airbag?

If your wife is properly belted in place with the lap belt and shoulder harness, the airbag isn't going to do anything.

Airbags were added to vehicles, at great expense, for the sole reason to ensure that the morons who do not use the belts won't get killed in a head on crash.

You could take your driver's airbag out of the SDL and be just as safe if you use the seat belt and shoulder harness.