HELP! can't change mail spool directory

hi all
i've been handed the task of managing an undocumented freebsd system which is 
in a bit of a mess!

the system is used for nonrepudiation of email and so all mails coming through 
exim are pushed out via procmail. the configuration i walked into has a 
/usr/local/etc/procmailrc which gives a global filter to all mails accounts. 
this is procmail v3.13.1.

i've just upped it to the latest release and found that the 
/urs/local/etc/procmailrc has no effect on all mails. at the moment i'm using a 
.procmailrc in a home directory - but i would also like to solve this problem 
as well!

anyway. on to the main problem...
the exim configuration has been using the standard unix mailbox store mechanism 
which is not very efficient for this particular implementation. i've rebuilt 
exim with support for maildir and pointed it at /var/mailbox as opposed to 
/var/mail

now in autoconf.h i changed the define MAILSPOOLDIR to reflect:

define MAILSPOOLDIR "/var/mailbox/"

but it keeps assigning /var/mail as the delivery point!
i cannot find ANYTHING, ANYWHERE to reflect this!

maybe i'm slightly confused about how procmail works..
in my exim configure i have the following transports and directors:

==== snip ====
local_delivery:
  driver = appendfile
  file = /var/mailbox/${local_part}
  create_directory = true
  maildir_format = true
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail
  mode = 0660

address_pipe:
  driver = pipe
  return_output

address_file:
  driver = appendfile
  delivery_date_add
  envelope_to_add
  return_path_add
  file = /var/mailbox/${local_part}
  create_directory = true
  maildir_format = true

userforward:
  driver = forwardfile
  file = .forward
  no_verify
  no_expn
  check_ancestor
# filter
  file_transport = address_file
  pipe_transport = address_pipe
  reply_transport = address_reply


# This director matches local user mailboxes.

localuser:
  driver = localuser
  transport = local_delivery
==== snip ====

now all the mails sent using this system are from local users to local users 
via SMTP and collected via POP3. when exim receives a mail, it does not write 
it to disc, it pipes it through procmail - which writes it to disc in 
/var/mail. or is this understanding wrong?

please. if you could help with:
a) the global /usr/local/etc/procmailrc problem
b) the /var/mailbox thing
c) anything more i need to use the maildir format

thanks for your time
mike