on my 1995 C250D the central locking module is beside the battery, you have to take the battery out to get at it. The Haynes manual says it is either there, or in the front passengers footwell. This is quite easy to get to, remove left side plastic trim & lift carpet, then remove metal plate held with 10mm plastic nuts. If you suspect something is drawing current when it shoudnt, it might be easier to connect a meter(on a low current range) in series with the battery & pull fuses. If you want more info on this, ask & ill post more.
PS nice to see some UK activity