The light monitor circuit looks for balanced currents between like bulbs (such as the tail light lamps).  If you replace any of the park light/tail light/brake light bulbs, replace them in pairs, to make sure you get the same type.  Even differing age of the bulbs can affect the pictogram.  Also, pay very close attention to the socket grounds if the pictogram starts acting up, particularly if it flashes when the turn signals or brakes are used.  You might want to avoid the use of sandpaper, as that tends to imbed particles in metal contacts.  A small wire brush in a Dremel tool should work well for cleaning lamp socket barrels.  Steel wool is good too.  Also, lightly dress the end contacts on the bulbs with a fine file.