My 1999 C280 has two "simple" remotes, with one big button and one small one. The small one pops the mechanical key out the side. The big one locks and unlocks the doors. However... it only works if aimed at and visible from the rear view mirror.

Does this mean the IR receiver in the door is defective?

The part number on the invoice was 210-760-26-06 and the description was "infrared key".

I programmed it by inserting the mechanical key, turning it from 0 to 2 to 0 twice, then removing it and holding it up to the sensor below the mirror and pushing the button five times.

This made the spare remote stop working.

One not-working mode is to fail to lock and unlock the doors. The other is to fail to keep the engine running.

Can both remotes be recognized, or is the system able only to recognize one at a time?