I have a 1988 300E and my defroster light on the dash will flash from time to time. Also once in a while the seat heaters will flash from time to time. Is this normal?

Normal but shouldn't be, Benz redesigned the charging system in 1990 & added an extra 15amps of power in the alternator! Most 1986-89 300E's suffer with this & other problems due to this. AT dealer where I work we always try to inform customers(who already need replacment) that for $50-60 more they can install the 80 amp alternator.