W251 remote central locking and radio reception not working

[-Lummox-]

Anybody have any experience of this?

My 2006 R320CDI will not lock and unlock using the key fob buttons via RF. It works unreliably via infrared though, sometimes on the first press of the button, sometimes more like the 100th press, doesn't seem to be any kind of consistent pattern with it.

Things I've checked/eliminated so far:

- Batteries in key fob x2 recplaced, same result with both
- New correct AGM battery fitted to car
- Aux battery inspected and fine
- All fuses inspected, none blown
- Alarm siren was dead but has been replaced with a new/genuine unit
- No fault codes showing that could explain it except possibly this one:
90C4 The Cable To Component A49/1(Interference Suppression Filter Has Short Circuit Or No Connection.

Any bright sparks got some ideas? I'm currently trying to source a replacement interfence supression filter but they seem to be discontinued, annoyingly. That was recommended as a next step on a dedicated Merc forum by someone who lost their radio reception (but not central locking) so may or may not be part of the problem.

Cheers!