it depends,
If there is two wires for the parking brake.... one needs to be grounded and the other goes to a switch so you can do the proper procedure(on-off-on(depending on what unit you have))manual will say which one.
Unless there is one wire then just ground it... but a switch under the dash is nice if the cops ever check.... and they do, so you can disable it before they get in.
**Disclaimer**
This is for information purposes only and does not reflect on any intentions or suggestions givin by this poster.... it is illegal to operate the monitor while the vehicle is in motion almost everywhere!!