I have just recently done what Ron Miller suggested with the 9v battery and the cigerette lighter plug. When you really need to disconect the power (removing the airbag in my case), this is the only solution. I was reminded to turn the key to the first (1) position. This worked for me, but I didn't test it or read the wiring diagram to see if this must be done.
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi