well if you don't have a fob the alarm will still work you open the door but your key in and the alarm light gos out only when its in the ignition
 
Chances are that a 300tdi wont have a chip key, but then again some cars after 1995 did have the immobiliser key.

Thus you might find that you'll have to reprogram the immobiliser if you change the key. But tbh it probably hasn't got a chipped key so it's nice n easy.

Cheers
 
96MY D1s onwards have a passive immobiliser fitted when new, therefore the fob has to be present near the ignition switch to remobilise the engine, the key doesn't have any electronics in it, it's just a key.
 

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