dynamic function question

I read the docs but I'm still a little lost as to how to do this.

I have this function which works fine.

CREATE OR REPLACE FUNCTION lookup_customer_byemail(varchar) RETURNS SETOF
customer_rec AS
'
DECLARE
r RECORD;
in_email ALIAS FOR $1;
BEGIN
FOR r IN SELECT
s_oid,mer_id,site_id,order_id,pymt_type,pymt_subtype,comp_name,f_name1,l_nam
e1,f_name2,l_name2,address,city,state
,zipcode,phone,cust_email,country from customers where cust_email ILIKE
in_email LOOP
RETURN NEXT r;
END LOOP;
RETURN;
END '
LANGUAGE 'plpgsql';

What I would like to do is pass in the column name that is being queried
against (cust_email) so it can be dynamic. I tried the following but it
always returns an empty set.

CREATE OR REPLACE FUNCTION lookup_customer(varchar,varchar) RETURNS SETOF
customer_rec AS
'
DECLARE
r RECORD;
in_string ALIAS FOR $1;
in_column ALIAS FOR $2;
BEGIN
FOR r IN SELECT
s_oid,mer_id,site_id,order_id,pymt_type,pymt_subtype,comp_name,f_name1,l_nam
e1,f_name2,l_name2,address,city,state
,zipcode,phone,cust_email,country from customers where in_column ILIKE
in_string LOOP
RETURN NEXT r;
END LOOP;
RETURN;
END '
LANGUAGE 'plpgsql';