If it's the factory alarm, you can disable it by pulling the lower of the two connectors on the alarm controller under the false firewall in the front passenger footwell. If you don't mind the extra step, unlock the door with the key, pull the door open, turn the key again to the unlock position. That should do it until you replace the switch in the door handle or disable the alarm completely.

If it's not the factory alarm, I don't know how to help you.

I'm interested to know what plastic you have to cut.

Sixto
91 300SE
81 300SD