which battery to disconnect?

[markm]

Just received my mirrors from powervision and plan the installation for this weekend. Powervision recommends disconnecting the negative terminal before beginning the mirror installation.
Which battery should I disconnect?
Will disconnecting the battery affect anything else other than resetting the security code on the stereo? :confused:

[IndigoDually]

Disconnect both batteries. They are wired in parallel. The system will still be energized if You only disconnect one. I believe that the clock will have to be reset after You unlock the radio. It locks when it loses power.

John