How can I label a Point using different postgis table

[Luis Treviño Huerta]

Hi list:

I'm currenttly using a point layer that displays some GPS points that are 
stored on a postgis table called "loca". I'd like to label this points with 
data that comes from another table in the same postgis data base (this table 
is named "autos"), both tables share a common column named "idunit" and I 
would like to use autos.idunit as a label. How can I do this?. This is part 
of my map file:

 LAYER
   NAME gis
   STATUS ON
   TYPE POINT
   UNITS METERS

   DEBUG ON
   CONNECTION "user=XXX password=XXXXX dbname=bsiubica host=XXX.XXX.XXX.XX 
port=5432"
   CONNECTIONTYPE postgis
   DATA "posicion from loca as foo using unique cmpoid using srid=4326"

   PROJECTION
      "proj=longlat"
      "ellps=WGS84"
     "datum=WGS84"
   END

  FILTER 'idunit = %unid%'

   CLASS
     EXPRESSION ('[mo]' = '1')
     COLOR 255 255 26  #amarillo
     SYMBOL 7
     SIZE 13
   END
END

regards,
Luis