I would think you need to disconnect the + and put the multi meter in series or as you say between battery and the disconnected lead.
Dont switch the ignition on, just check it dead then check again with the RNSE switched on. dont switch anything else on as you might draw more amps than your multi can take. Remember to shut the door etc so the interior lights are out, you dont want anything other than the radio drawing current then switch it off and see what the reading is. the RNSE will always draw a current because it has a memory but TBH its your alarm that is probably the big draw.
Dont switch the ignition on, just check it dead then check again with the RNSE switched on. dont switch anything else on as you might draw more amps than your multi can take. Remember to shut the door etc so the interior lights are out, you dont want anything other than the radio drawing current then switch it off and see what the reading is. the RNSE will always draw a current because it has a memory but TBH its your alarm that is probably the big draw.