Okay so the theory is that if you lock your convertible, someone will slash the top to get in. So, you should set the alarm and not lock the doors. And, you should not leave anything valuable in the car.

I have not figured out how to set the alarm without using the keyfob which automatically locks the doors. Can anyone help?

When you set the alarm on a c70 the doors lock...........dead bolt. Ie: one can not lock the doors without the alarm being activated.