You need a dictionary to translate POS NLTK tags into WordNet tags:
pos_translate = {'J':'a', 'V':'v', 'N':'n', 'R':'r'}
, POS, , ( , , , "n" lemmatize:
text = ['This', 'is', 'a', 'car', '.']
[lemmatizer.lemmatize(w,\
pos=pos_translate[pos[0]] if pos[0] in pos_translate else 'n')\
for w,pos in nltk.pos_tag(text)]