I had familiar problem with my alarm on 420sel. I ended up disconnecting alarm module. Its located in passenger foot area. Lift passenger side carpets. Youll see plastic cover like fake wall. There is plasic nut or screw in the center bottow of cover. Remove it. Then remove plastic cover. You will see couple of black boxes and O2 sensor connector. On mine alarm module was marked as Bekker Autoradio(I know its weird but it was alarm module). There is plug on its side. Disconnect it and alarm will disable. I had no problems with my radio nor central locking system with module disconnected. I hope it helps.
Good luck

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1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod